UVa - 575 - Skew Binary

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
int main(){
    long int n, i, sum, num, b;
    char s[100];
    while(gets(s)){
            sum = 0;
            n = strlen(s);
            for(i = 0; i < n; i++){
                    num = s[i] - '0';
                    b = num * (pow(2, n - i) - 1);
                    sum = sum + b;
                    }
                    if(sum == 0)break;
                    printf("%ld\n", sum);
                    }
                    return 0;
                    }