#include<bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<int, int >PII;

const int N = 100;

int w[30][30];
int h[30] = {0};
int n, m;
int dx[4] = { -1, 0, 1, 0 }, dy[4] = { 0, 1, 0, -1 };

int ans = 0;
void dfs(int x, int y ,int step)
{
    h[w[x][y]] = true;
    for (int i = 0; i < 4; i++)
    {
        int a = x + dx[i], b = y + dy[i];       //判断下个点在范围内且没有走过
        if (a < 0 || a >= n || b < 0 || b >= m)continue;
        if (h[w[a][b]])continue;
        h[w[a][b]] = true;
        dfs(a, b, step + 1);
        h[w[a][b]] = false;
    }
    ans = max(ans, step);
}


int main() {
    //ios::sync_with_stdio; 
    cin >> n >> m;
    char a;
    for(int i=0;i<n;i++)
        for (int j = 0; j < m; j++)
        {
            cin >> a;
            w[i][j] = a-65;
        }

    dfs(0, 0, 1);
    cout << ans << endl;
    return 0;

}