1_2 !falied! mulmod Gorner (working)

#include <stdio.h>
int main()
{
    int i = 0, b[64];
    unsigned long a, s, c, m;
    scanf ("%lu%lu%lu", &a, &c, &m);

    for (i = 0; i <= 63; i++) {
        b[i] = c & 1;
        c = c>>1;
    }
    s = a * b[63];

    for (i = 63; i >= 0; i--) {
        s = (a * b[i] % m) + (2 * s % m);
    }

    s = s % m;
    printf ("%lu", s);
    return 0;
}