#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <cstring>
#include <cmath>
using namespace std;
const int dx[] = { 0,1,0,-1 };
const int dy[] = { 1,0,-1,0 };
char arr[20][20] = { 0 };
int r, c;
int alpha[150] = { 0 };
int ans = 1;

void dfs(int x,int y,int cnt) {
    if (cnt > ans)
        ans = cnt;
    for (int dir = 0; dir < 4; dir++) {
        int nx = x + dx[dir];
        int ny = y + dy[dir];
        if (nx >= 0 && nx < r&&ny >= 0 && ny < c&&alpha[arr[nx][ny]] == 0) {
            alpha[arr[nx][ny]] = 1;
            dfs(nx, ny, cnt + 1);
            alpha[arr[nx][ny]] = 0;
        }
    }
}

int main() {
    cin >> r >> c;
    char tmp;
    scanf("%c", &tmp);
    for (int i = 0; i < r; i++) {
        for (int j = 0; j < c; j++) {
            scanf("%1c", &arr[i][j]);
        }
        scanf("%c", &tmp);
    }
    alpha[arr[0][0]] = 1;
    dfs(0, 0, 1);
    cout << ans;
    return 0;
}