#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
char a[30][30];
int num[30];
int vis[30][30];
int maxx = 0;
int r, s;
int dir[4][2] = { {0,1},{0,-1},{1,0},{-1,0} };
void dfs(int x, int y, int step)
{
	if (maxx < step)
	{
		maxx = step;
	}
	for (int i = 0; i < 4; i++)
	{
		
		int nx = x + dir[i][0];
		int ny = y + dir[i][1];
		if (nx >= 0 && nx < r && ny >= 0 && ny < s && vis[nx][ny] == 0 && num[a[nx][ny] - 'A'] == 0)
		{
			vis[nx][ny] = 1;
			num[a[nx][ny] - 'A'] = 1;
			dfs(nx, ny, step + 1);
			vis[nx][ny] = 0;
			num[a[nx][ny] - 'A'] = 0;
		}
	}
}
int main()
{
	cin >> r >> s;
	for (int i = 0; i < r; i++)
		for (int j = 0; j < s; j++)
			cin >> a[i][j];
	num[a[0][0] - 'A'] = 1;
	vis[0][0] = 1;
	dfs(0, 0, 1);
	cout << maxx << endl;
	return 0;
}