#include<queue>
#include<math.h>
#include<stdio.h>
#include<iostream>
#include<vector>
#include<iomanip>
#include<string.h>
#include<algorithm>
#include<cmath>
#include<cstdio>
#include<cstring>
#include<stack>
#include <fstream>
#include<string>
using namespace std;
#define LL long long
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
int R, C;
vector<string> board;
int max_count = 0;
bool visited[26] = { false };
void dfs( int x , int y, int count)
{
    max_count = max( max_count , count );
    int dx[] = { -1 , 1 , 0 , 0 };
    int dy[] = { 0 , 0 , -1 , 1 };    
    for ( int i = 0 ; i < 4 ; i++ )
	{
        int nx = x + dx[i];
        int ny = y + dy[i];
        if ( nx >= 0 && nx < R && ny >= 0 && ny < C )
		{
            char c = board[nx][ny];
            if ( !visited[ c - 'A' ] )
			{
                visited[ c - 'A' ] = true;
                dfs( nx , ny , count + 1 );
                visited[ c - 'A' ] = false;
            }
        }
    }
}
int main()
{
    cin >> R >> C;
    board.resize(R);
    for ( int i = 0 ; i < R ; i++ )
	{
        cin >> board[i];
    }
    visited[ board[0][0] - 'A' ] = true;
    dfs( 0 , 0 , 1 );    
    cout << max_count << endl;
    return 0;
}