How do I approach further in the following problem?

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Python3.6

Information about courses
Line format: Course Code~Course Name~Semester~Year~Instructor
Information about students
Line format: Roll Number~Full Name
Information about grades
Line format: Course Code~Semester~Year~Roll Number~Grade
The possible grades are A, AB, B, BC, C, CD, D with corresponding grade points 10, 9, 8, 7, 6, 5 and 4. The grade point average of a student is the sum of his/her grade points divided by the number of courses. For instance, if a student has taken two courses with grades A and C, the grade point average is 8.50 = (10+7)÷2. If a student has not completed any courses, the grade point average is defined to be 0.

You may assume that the data is internally consistent. For every grade, there is a corresponding course code and roll number in the input data.

Each section of the input starts with a line containing a single keyword. The first section begins with a line containing Courses. The second section begins with a line containing Students. The third section begins with a line containing Grades. The end of the input is marked by a line containing EndOfInput.

Write a Python program to read the data as described above and print out a line listing the grade point average for each student in the following format:

Roll Number~Full Name~Grade Point Average
Your output should be sorted by Roll Number. The grade point average should be rounded off to 2 digits after the decimal point. Use the built-in function round().

Here is a sample input and its corresponding output.

Sample Input

Courses
TRAN~Transfiguration~1~2011-2012~Minerva McGonagall
CHAR~Charms~1~2011-2012~Filius Flitwick
Students
SLY2301~Hannah Abbott
SLY2302~Euan Abercrombie
SLY2303~Stewart Ackerley
SLY2304~Bertram Aubrey
SLY2305~Avery
SLY2306~Malcolm Baddock
SLY2307~Marcus Belby
SLY2308~Katie Bell
SLY2309~Sirius Orion Black
Grades
TRAN~1~2011-2012~SLY2301~AB
TRAN~1~2011-2012~SLY2302~B
TRAN~1~2011-2012~SLY2303~B
TRAN~1~2011-2012~SLY2305~A
TRAN~1~2011-2012~SLY2306~BC
TRAN~1~2011-2012~SLY2308~A
TRAN~1~2011-2012~SLY2309~AB
CHAR~1~2011-2012~SLY2301~A
CHAR~1~2011-2012~SLY2302~BC
CHAR~1~2011-2012~SLY2303~B
CHAR~1~2011-2012~SLY2305~BC
CHAR~1~2011-2012~SLY2306~C
CHAR~1~2011-2012~SLY2307~B
CHAR~1~2011-2012~SLY2308~AB
EndOfInput


Sample Input

SLY2301~Hannah Abbott~9.5
SLY2302~Euan Abercrombie~7.5
SLY2303~Stewart Ackerley~8.0
SLY2304~Bertram Aubrey~0
SLY2305~Avery~8.5
SLY2306~Malcolm Baddock~6.5
SLY2307~Marcus Belby~8.0
SLY2308~Katie Bell~9.5
SLY2309~Sirius Orion Black~9.0

What I have tried:

(lis1, lis2, lis3)= ([], [], [])
 
a = input()
while True:
	
	if a == 'Courses':
		a =input()
		while a != 'Students':
			lis1.append(a.split('~'))
			a = input()
	elif a == 'Students':
		a =input()
		while a!= 'Grades':
			a.split(' ')
			lis2.append(a.split('~'))
			a = input()
	elif a == 'Grades':
		a =input()
		while a != 'EndOfInput':
			lis3.append(a.split('~'))
			a = input()
	elif a == 'EndOfInput':
		break
	else:
		break

i have been successful in making separate list of Course(lis1), Students(lis2), Grades(lis3) but i Don't know what to do next and how to approach further plz help!!. I guess i require dictionaries too. also how can i calculate gradepoint from letters and how to finally join al the specific values. I gues Join method of python shall be helpful but i am not getting how to arrange all parameters