#include <stdio.h>

main(){
  printf("Enter hex number");

  int result = getchar();

  result  = atoi(result);

  printf("the string is " + result);

}

int atoi(char s[]){
  int i, n;
  n = 0;
  for (i = 0; s[i] != '\0'; ++i){
    if (s[i] == 'a')
      n+= 10;
    else if (s[i] == 'b')
      n+= 11;
    else if (s[i] == 'c')
      n+= 12;
    else if (s[i] == 'd')
      n+= 13;
    else if (s[i] == 'e')
      n+= 14;
    else if (s[i] == 'f')
      n+= 15;
    else
      n+=s[i];
  }
  return n;
}