#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int n, k;
    cin >> n >> k;

    vector<char> v(n);

    for(int i = 0; i < n; i++)
    {
        cin >> v[i];
    }

    vector<int> s(n + 1);

    for(int i = n - 1; i >= 0; i--)
    {
        s[i] = s[i + 1];   
        if(v[i] == '1') s[i]++; 
    }

    long long ans = 0;

    int cnt = 0;
    int j = 0;

    for(int i = 0; i < n; i++)
    {
        if(v[i] == '1')
        {
            while(j < n && cnt != k)
            {
                if(v[j] == '1') cnt++;
                j++;
            }

            if(cnt == k) ans += s[j - 1];

            cnt--;
        }
    }

    cout << ans;

    return 0;
}