#include <bits/stdc++.h>
#define int long long
using namespace std;
 
int n, k; 
int psum[100005];
 
bool check(int d) { // kiểm tra độ dài d có hợp lệ ko
    for(int i = 1; i + d - 1 <= n; ++ i) {
        int cnt = psum[i + d - 1] - psum[i - 1];
        if(cnt < k) return false;
    }
    return true;
}
 
int32_t main() {
    cin >> n >> k;
    string s; cin >> s;
    for(int i = 1; i <= n; ++ i) { // tính mảng cộng dồn để đếm số cây trong đoạn
        psum[i] = psum[i - 1];
        if(s[i - 1] == '#') ++ psum[i];
    }
 
    int l = 1, r = n, d = -1;
    while(l <= r) { // tìm d bằng tìm kiếm nhị phân
        int mid = (l + r) / 2;
        if(check(mid)) {
            d = mid;
            r = mid - 1;
        }
        else l = mid + 1;
    }
    cout << d;
}

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