#include <iostream>
#include <math.h>

using namespace std;

int main(){

    double x;
    int i;

    cin >> x;

    x = fmod (x, 7.0);

    printf("%.0f",x);

    system("pause");
    return 0;

}

Edited by author 18.08.2014 21:49

it's ovbious that a number of 50 digits won't fit in a double. Such number wouldn't even fit in long long. You need other method...