#include <bits/stdc++.h>

using i64 = long long;
using u64 = unsigned long long;
using u32 = unsigned;
using u128 = unsigned __int128;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    int n, m;
    std::cin >> n >> m;
    
    int ans = 0;
    
    std::vector<std::string> s(n);
    for (int i = 0; i < n; i++) {
        std::cin >> s[i];
    }
    
    auto work = [&] {
        std::vector<int> pre(n + 1), suf(n + 1);
        std::vector<int> h(n);
        for (int i = 0; i < n; i++) {
            int res = pre[i];
            for (int j = 0, l = -1; j < n; j++) {
                if (s[i][j] == 'X') {
                    l = j;
                    h[j] = 0;
                } else {
                    h[j]++;
                    res = std::max({res, j - l, h[j]});
                }
            }
            pre[i + 1] = res;
        }
        std::fill(h.begin(), h.end(), 0);
        for (int i = n - 1; i >= 0; i--) {
            int res = suf[i + 1];
            for (int j = 0, l = -1; j < n; j++) {
                if (s[i][j] == 'X') {
                    l = j;
                    h[j] = 0;
                } else {
                    h[j]++;
                    res = std::max({res, j - l, h[j]});
                }
            }
            suf[i] = res;
        }
        if (m == 1) {
            ans = std::max(ans, pre[n]);
        } else {
            for (int i = 0; i <= n; i++) {
                ans = std::max(ans, std::min(pre[i], suf[i]));
            }
        }
    };
    
    work();
    {
        std::vector<std::string> ns(n);
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                ns[n - 1 - j] += s[i][j];
            }
        }
        s = std::move(ns);
    }
    work();
    
    std::cout << ans << "\n";
    
    return 0;
}