conversions from one base to another

// This program converts given no. to decimal.
// then it converts decimal no. to source base.
//checks if the new no. and initial no. are same
//if same, converts the decimal no. into target base
//if not, again asks for input
#include<stdio.h>
#include<conio.h>
#include<math.h>          //for pow function
#include<string.h>        //for strcmpi

long int cnvrtdeci(char *, int);              //convert into decimal
int chkbase(long int, int, char*);            //checks base
char* cnvrt(long int,int);                    //convert into target base

void main()
{
    char num[11],*p;
    long int m;
    int base=0,trgt, c,i;
    clrscr();
    back:
    printf("\nInput:");
    scanf("%s", num);
    scanf("%d %d", &base, &trgt);
    m=cnvrtdeci(&num[0], base);
    c=chkbase(m, base, &num[0]);
    if(c==1)
    {
       printf("\nWrong input!!");
       goto back;
    }
    p = cnvrt(m,trgt);
    printf("\nOutput:");
    for (; *p!='\0';p++)
       printf("%s", *p);
    getch();
}
long int cnvrtdeci(char *j,int b)
{
    char *i;
    long int sum=0;
    int k;
    while( *j != '\0' )
      j++;
    for( i=j-1,k=0; i>=0; i--,k++)
    {
      switch(*i)
      {
        case 'A':
        case 'a': sum+=10 * pow(b,k);
              break;
        case 'B':
        case 'b': sum+=11 * pow(b,k);
              break;
        case 'C':
        case 'c': sum+=12 * pow(b,k);
              break;
        case 'D':
        case 'd': sum+=13 * pow(b,k);
              break;
        case 'E':
        case 'e': sum+=14 * pow(b,k);
              break;
        case 'F':
        case 'f': sum+=15 * pow(b,k);
              break;
        case 'G':
        case 'g': sum+=16 * pow(b,k);
              break;
        case 'H':
        case 'h': sum+=17 * pow(b,k);
              break;
        case 'I':
        case 'i': sum+=18 * pow(b,k);
              break;
        case 'J':
        case 'j': sum+=19 * pow(b,k);
              break;
        default: sum+= (*j) * pow(b,k);
       }

    }
    return sum;
}
int chkbase( long int m,int base,char *p)
{
    char *j;
    int t;
    j= cnvrt(m,base);
    t= strcmpi ( j,p);
    if( t!=0)
         return 1;
    else
         return 0;
}

char* cnvrt( long int m, int t)
{
    char res[50];
    int r,j;
    for( j=48;m>=0;j--)
    {
      r= m%t;
      m= m/t;
      if ( r>=0 && r<=9 )
       res[j]=r;
      else
       switch(r)
       {
         case '10': res[j] = 'A';
            break;
         case '11': res[j] = 'B';
            break;
         case '12': res[j] = 'C';
            break;
         case '13': res[j] = 'D';
            break;
         case '14': res[j] = 'E';
            break;
         case '15': res[j] = 'F';
            break;
         case '16': res[j] = 'G';
            break;
         case '17': res[j] = 'H';
            break;
         case '18': res[j] = 'I';
            break;
         case '19': res[j] = 'J';
            break;
         }
      }
         return &res[0];
}