#include<iostream>using namespace std;const int Max = 30;int row, col, ans = 0;char map[Max][Max];bool vis[Max];int dr[4] = {-1, 0, 0, 1};int dc[4] = {0, -1, 1, 0};bool inmap(int r, int c){ if(r >= 1 && r <= row && c >= 1 && c <= col) return true; return false;}void dfs(int dep, int now_r, int now_c){ if(dep > ans) ans = dep; for(int i = 0; i < 4; i ++){ int r = now_r + dr[i]; int c = now_c + dc[i]; if(inmap(r, c) && !vis[map[r][c] - 'A']){ vis[map[r][c] - 'A'] = true; dfs(dep + 1, r, c); vis[map[r][c] - 'A'] = false; } }}int main(){ cin >> row >> col; for(int i = 1; i <= row; i ++) for(int j = 1; j <= col; j ++) cin >> map[i][j]; memset(vis, false, sizeof(vis)); vis[map[1][1] - 'A'] = true; dfs(1, 1, 1); cout << ans << endl; return 0;}
