#include<iostream>
#include<queue>
using namespace std;


int r,c;
char map[100][100];
int move[4][2]={{0,1},{0,-1},{1,0},{-1,0}};



struct note
{
	int x;
	int y;
	int count;
	bool used[26];
};


void bfs()
{
	int max=0;
	queue<note> q;
	int front=0,rear=0;
	note current,next;
	current.x=0,current.y=0;
	current.count=1;
	memset(current.used,false,sizeof(current.used));
	current.used[map[0][0]-'A']=true;
	q.push(current);
	
	
	
	while(!q.empty())
	{	
		current=q.front();
		q.pop();
		if(current.count>max)
			max=current.count;
		
		int i;
		for(i=0;i<4;i++)
		{		
			next.x=current.x+move[i][0];
			next.y=current.y+move[i][1]; 
			int j;
			for(j=0;j<26;j++)
			{
				next.used[j]=current.used[j];
			}
			if(next.x>=0&&next.x<r&&next.y>=0&&next.y<c&&!next.used[map[next.x][next.y]-'A'])
			{
				next.count=current.count+1;
				next.used[map[next.x][next.y]-'A']=true;
				q.push(next);
			}
		}
	}
	printf("%d\n",max);
}


int main()
{
	scanf("%d%d",&r,&c);
	int i;
	for(i=0;i<r;i++)
	{
		cin>>map[i];
	}
	
	bfs();
	return 0;
}