#include <bits/stdc++.h>
using namespace std;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> wf;
for (int i = 0; i < n; ++i)
{
if (s[i] == '1')
{
wf.push_back(i + 1);
}
}
long long cost = 0;
int laco = 0;
int wfidx = 0;
int m = wf.size();
if (m == 0)
{
cost = (long long)n * (n + 1) / 2;
cout << cost << '\n';
return 0;
}
for (int i = 1; i <= n; )
{
if (i <= laco)
{
i++;
continue;
}
while (wfidx < m && wf[wfidx] < i - k)
{
wfidx++;
}
if (wfidx < m && wf[wfidx] <= i + k)
{
int pos = wf[wfidx];
cost += pos;
laco = min(pos + k, n);
i = laco + 1;
wfidx++;
}
else
{
cost += i;
laco = i;
i++;
}
}
cout << cost << "\n";
return 0;
}