count number of 101 in a string

Here is the question: we need to count number of 101 in a string composed of 0s and 1s only. 101 is any subsequence of the string.

Ex: 10101 has 4 101 in it.

I'm pretty sure that I solved it correctly. For each zero I precalculate number of 1 before it and after it then multiply the answers then add the result to res.

The code is giving me wrong answer for a test case composed of a string of length 1000000.

I would like to know where is the problem in my code?

Input for the Test Case: http://ift.tt/1C9AISJ

Output shoud be 18446708952791232852 but my code is giving 22531786

Here is my code:

char s[1000005];

unsigned long long ans, res, a[1000005];

int main()
{   
    int n;

    scanf("%s", s);
    n = strlen(s);

    a[0] = 0; res = 0;
    for(int i = 1;i <= n;++i)
        a[i] = a[i - 1] + (s[i - 1] == '1');

    for(int i = 1;i <= n;++i)
        if(s[i - 1] == '0') {
            ans = a[n] - a[i];
            ans *= a[i - 1];
            res += ans;
            //if(ans < 0 || res < 0) printf("%lld %lld\n", ans, res);
        }

    printf("%llu\n", res);


    return 0;
}