The Skill Level Goes Into A Negative Number When I Have Assigned It To Zero.

import random
import time
skillnumber = []
strengthnumber = []
character1 = raw_input("What would you like to call your character?")
dicefour = [1,2,3,4]
dicesix = [1,2,3,4,5,6]
dicetwelve = [1,2,3,4,5,6,7,8,9,10,11,12]
skill = 10
strength = 10
dicefourscore = random.choice(dicefour)
dicetwelvescore = random.choice(dicetwelve)
attribute = int(input("Please choose an attribute, 1:Strength, 2:Skill"))
if attribute == 1:
        totalscore = dicetwelvescore // dicefourscore
        strength1 = totalscore + strength
        print ("%s's strength level is now %s") % (character1, strength1)
        strengthnumber.append(strength1)
        print ("Let's determine the character's skill level")
        time.sleep(1)
        dicefourscore = random.choice(dicefour)
        dicetwelvescore = random.choice(dicetwelve)
        totalscore = dicetwelvescore // dicefourscore
        skill1 = skill + totalscore
        print ("%s's skill level is now %s") % (character1, skill1)
        skillnumber.append(skill1)
elif attribute == 2:
        totalscore = dicetwelvescore % dicefourscore
        skill1 = skill + totalscore
        print ("%s's skill level is now %s") % (character1, skill1)
        skillnumber.append(skill)
        print ("Let's determine the character's strength level")
        time.sleep(2)
        dicefourscore = random.choice(dicefour)
        dicetwelvescore = random.choice(dicetwelve)
        totalscore = dicetwelvescore // dicefourscore
        strength1 = strength + totalscore
        print ("%s's strength level is now %s") % (character1, strength1)
        strengthnumber.append(strength)
elif attribute != 1 and attribute != 2:
        print ("Could you either choose one of the attributes skill or strength")
character2 = raw_input("What would you like to call your character?")
dicefour = [1,2,3,4]
dicetwelve = [1,2,3,4,5,6,7,8,9,10,11,12]
skill = 10
strength = 10
dicefourscore = random.choice(dicefour)
dicetwelvescore = random.choice(dicetwelve)
attribute = int(input("Please choose an attribute, 1:Strength, 2:Skill"))
if attribute == 1:
        totalscore = dicetwelvescore // dicefourscore
        strength2 = totalscore + strength
        print ("%s's strength level is now %s") % (character2, strength2)
        strengthnumber.append(strength2)
        print ("Let's determine the character's skill level")
        time.sleep(1)
        dicefourscore = random.choice(dicefour)
        dicetwelvescore = random.choice(dicetwelve)
        totalscore = dicetwelvescore // dicefourscore
        skill2 = skill + totalscore
        print ("%s's skill level is now %s") % (character2, skill2)
        skillnumber.append(skill2)
elif attribute == 2:
        totalscore = dicetwelvescore % dicefourscore
        skill2 = skill + totalscore
        print ("%s's skill level is now %s") % (character2, skill2)
        skillnumber.append(skill2)
        print ("Let's determine the character's strength level")
        time.sleep(2)
        dicefourscore = random.choice(dicefour)
        dicetwelvescore = random.choice(dicetwelve)
        totalscore = dicetwelvescore // dicefourscore
        strength2 = strength + totalscore
        print ("%s's strength level is now %s") % (character2, strength2)
        strengthnumber.append(strength2)
elif attribute != 1 and 2:
        print ("Could you either choose one of the attributes skill or strength")
if strength1 < strength2:
      difference = strength2-strength1
      strengthmodifier = difference // 5
      if strengthmodifier == 0:
        strengthmodifier = strengthmodifier + 1
      print (" the strength modifier is %s") % (strengthmodifier)

if strength1 > strength2:
     difference = strength1-strength2
     strengthmodifier = difference // 5
     if strengthmodifier == 0:
        strengthmodifier = strengthmodifier + 1
     print (" the strength modifier is %s") % (strengthmodifier)
if strength1 == strength2:
     print (" The strength modifier is 1")   # problems when modifier is 0
if skill1 == skill2:
    print (" the skill modifier is 1")
if skill1 < skill2:
      difference = skill2-skill1
      skillmodifier = difference // 5
      if skillmodifier == 0:
        skillmodifier = skillmodifier+1
      print (" the skill modifier is %s") % (skillmodifier)
if skill1 > skill2:
     difference = skill1 - skill2
     skillmodifier = difference // 5
     if skillmodifier == 0:
        skillmodifier = skillmodifier + 1
     print (" the skill modifier is %s") % (skillmodifier)

character1dice = random.choice(dicesix)
print character1dice
character2dice = random.choice(dicesix)
print character2dice

while 1 == 1:

    if skill1 < 0:
       skill1 = 0

    if skill2 < 0:
       skill2 = 0

    if strength1 <= 0:
        print ("%s is dead") %(character1)
        break

    if strength2 <= 0:
        print ("%s is dead") %(character2)
        break
    while skill1>=0 and skill2>=0 and strength1>0 and strength2>0:
        if character1dice > character2dice:
            strength1 = strength1 + strengthmodifier
            skill1 = skill1 + skillmodifier
            strength2 = strength2 - strengthmodifier
            skill2 = skill2 - skillmodifier
            print ("%s's strength level is %s") %(character1,strength1)
            print ("%s's skill level is %s") %(character1,skill1)
            print ("%s's strength level is %s") %(character2,strength2)
            print ("%s's skill level is %s") %(character2,skill2)

        if character2dice > character1dice:
            strength2 = strength2 + strengthmodifier
            skill2 = skill2 + skillmodifier
            strength1 = strength1 - strengthmodifier
            skill1 = skill1 - skillmodifier
            print ("%s's strength level is %s") %(character1,strength1)
            print ("%s's skill level is %s") %(character1,skill1)
            print ("%s's strength level is %s") %(character2,strength2)
            print ("%s's skill level is %s") %(character2,skill2)

        if character1dice == character2dice:
            print ("%s's strength level is %s") %(character1,strength1)
            print ("%s's skill level is %s") %(character1,skill1)
            print ("%s's strength level is %s") %(character2,strength2)
            print ("%s's skill level is %s") %(character2,skill2)