Letters

#include <bits/stdc++.h>

using namespace std;

int n, m;
char grid[21][21];
bool visited[27];
int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, 1, -1};
int ans;

void dfs(int x, int y, int counter){
    if(visited[grid[x][y]-'A'] || x<0 || x>=n || y<0 || y>=m){
        return;
    }
    //visited[x][y] = true;
    //counter++;
    visited[grid[x][y]-'A'] = true;
    ans = max(ans, counter);
    for(int i=0; i<4; i++){
        int nx = x+dx[i];
        int ny = y+dy[i];
        if(!visited[grid[nx][ny]-'A'] && nx>=0 && nx<n && ny>=0 && ny<m){
            counter++;
            //cout << "nx: " << nx << ", ny: " << ny << ", grid[nx][ny]: " << grid[nx][ny] << ", counter: " << counter << '\n';
            //visited[grid[x][y]-'A'] = true;a
            dfs(nx, ny, counter);
            counter--;
            //visited[grid[x][y]-'A'] = false;
        }
    }
    //counter--;
    visited[grid[x][y]-'A'] = false;
}

int main(){
    //freopen("letters.in", "r", stdin);

    cin >> n >> m;
    string s;
    for(int i=0; i<n; i++){
        cin >> s;
        for(int j=0; j<m; j++){
            grid[i][j] = s[j];
        }
    }
    //for(int i=0; i<n; i++){
        //for(int j=0; j<m; j++){
            //cout << "i: " << i << ", j: "<< j << ", grid[i][j]: " << grid[i][j] << '\n';
            //memset(visited, false, sizeof(visited));
            //dfs(i, j, 1);
        //}
    //}
    dfs(0, 0, 1);
    cout << ans << '\n';

    return 0;
}