#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 1;
vector<long long> arr(2e5 + 2, 0);
vector<long long> dp(2e5 + 2, 0);
vector<long long> left1(2e5 + 2, 0);
void solve() {
  int n, k;
  string s;
  cin >> n >> k >> s;
  for (int i = 0; i < n; i++) {
    arr[i] = (s[i] - '0') * k;
  }
  long long last = n;
  for (int i = n; i >= 0; i--) {
    left1[i] = last;
    if (i - 1 >= 0 && arr[i - 1] != 0) last = i - 1;
  }
  dp[0] = 1;
  for (int i = 1; i < n; i++) {
    long long firstcost =
        (i - 1 - arr[i] >= 0) ? (dp[i - 1 - arr[i]] + i + 1) : (i + 1);
    firstcost = min(firstcost, dp[i - 1] + i + 1);
    long long rr = left1[i + 1];
    if (rr < n && rr - i <= k) {
      firstcost = min(firstcost,
                      (rr - k - 1 >= 0) ? (dp[rr - k - 1] + rr + 1) : (rr + 1));
    }
    long long l = max(i - k - 1, -1);
    long long r = left1[l + 1];
    if (r < i) {
      firstcost =
          min(firstcost, ((r - k - 1 >= 0) ? dp[r - k - 1] + r + 1 : (r + 1)));
    }
    dp[i] = firstcost;
  }
  cout << dp[n - 1];
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  solve();
  return 0;
}
