#include <stdio.h>
#include <ctype.h>

int expo(int x, int y);
int atoi(char c);
int htoi(char s[]);

void main() {
  printf("%d\n", htoi("62Fa"));
}

int expo(int x, int y){
  int res;
  int i;
  
  if(y > 0) {
    res = x;
    for(i = 1; i < y; ++i){
      res = res * x;
    }
  } else if(y == 0){
    res = 1;
  }
  
  return res;
}

int atoi(char c){
  return c - '0';
}

int htoi(char s[]) {
  int tail = 0;
  int head = 0;
  int res = 0;
    
  while(s[tail] != '\0'){
    ++tail;
  }
  
  while(tail > 0) {
    if(isdigit(s[head])){
      res = res + (atoi(s[head]) * expo(16, tail-1));
    }else if(s[head] >= 65 && s[head] <= 70){
      res = res + ((s[head] - 55) * expo(16, tail-1));
    }else if(s[head] >= 97 && s[head] <= 102){
      res = res + ((s[head] - 87) * expo(16, tail-1));
    }
    
    --tail;
    ++head;
  }
  
  return res;
}