gistfile1.txt

// Maze Master

#include <iostream>
#include <vector>
#include <math.h>
#include <string>
#include <map>
#include <stdio.h>
#include <string.h>
#include <initializer_list>
#include <algorithm>
#include <numeric>
#include <iomanip>
#include <queue>
#include <set>

using namespace std;

const int INF = 1 << 29;

int main ()
{
    int H,W;
    cin >> H >> W;
    char S[20][20];
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            cin >> S[i][j];
        }
    }
    
    int dp[20][20][20][20];
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {

            for (int k = 0; k < H; k++) {
                for (int l = 0; l < W; l++) {

                    dp[i][j][k][l] = INF;

                    
                    if (i == k && j == l) {
                        dp[i][j][k][l] = 0;
                    }
                    
                }
            }
        }
    }

    int dx[4] = {1, -1, 0, 0};
    int dy[4] = {0, 0, 1, -1};

    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            if (S[i][j] == '#') {
                continue;
            }


            for (int k = 0; k < 4; k++) {
                int nx = dx[k] + j;
                int ny = dy[k] + i;
                if (nx < 0 || ny < 0 || nx >= W || ny >= H) {
                    continue;
                }
                if (S[ny][nx] == '#') {
                    continue;
                }
                dp[ny][nx][i][j] = dp[i][j][ny][nx] = 1;
            }
        }
    }

    int cost = 0;
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {

            for (int k = 0; k < H; k++) {
                for (int l = 0; l < W; l++) {

                    for (int m = 0; m < H; m++) {
                        for (int n = 0; n < W; n++) {

                            dp[k][l][m][n] = min(dp[k][l][m][n], dp[k][l][i][j] + dp[i][j][m][n]);
                        }
                    }

                }
            }

        }
    }

    /*
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            for (int k = 0; k < H; k++) {
                for (int l = 0; l < W; l++) {
                    cout << dp[i][j][k][l] << " ";
                }
            }
            cout << endl;
        }
    }
    */
    
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {

            for (int k = 0; k < H; k++) {
                for (int l = 0; l < W; l++) {
                    if (dp[i][j][k][l] < INF) {
                        cost = max(cost, dp[i][j][k][l]);
                    }
                }
            }

        }
    }

    cout << cost << endl;

    return 0;
}