#include <iostream>
#include <cstdio>
#include<algorithm>
#include<vector>
using namespace std;
int R, C, m=0;
char a;
int cnt = 0;
char map[20][20];
bool visited[20][20], al[26];
vector<int> v;
int dir[4][2] = { { -1,0 },{ 0,1 },{ 1,0 },{ 0,-1 } };

void dfs(int x, int y, int cnt) {

    m = max(m, cnt);

    for (int i = 0; i < 4; i++) {

        int nx = x + dir[i][0];

        int ny = y + dir[i][1];

        if (0 <= nx && nx < R && 0 <= ny && ny < C) {

            if (visited[nx][ny] == false && al[(int)map[nx][ny] - 65] == false) {

                visited[nx][ny] = true;

                al[(int)map[nx][ny] - 65] = true;

                dfs(nx, ny, cnt+1);

                visited[nx][ny] = false;

                al[(int)map[nx][ny] - 65] = false;    //백트래킹
            }
        }
    }

}

int main(void)
{
    cin.tie(NULL);
    cout.tie(NULL);

    cin >> R >> C;      //R:행의 개수, C:열의 개수
    for (int i = 0; i < R; i++) {
        for (int j = 0; j < C; j++) {
            cin >> a;
            map[i][j] = a;
        }
    }

    visited[0][0] = true;

    al[(int)map[0][0] - 65] = true;

    dfs(0, 0, 1);

    cout << m;

}