LETTERS

//LETTERS
#include<bits/stdc++.h>
using namespace std;
int u[4]={-1,0,1,0};
int v[4]={0,1,0,-1};
bool b[27];
char a[21][21];
int r,s,max1;
int search(int m,int n,int k)
{
if(k>max1)max1=k;
for(int i=0;i<=3;i++)
{
int x=m+u[i];int y=n+v[i];
if(x>=1&&x<=r&&y>=1&&y<=s&&!(b[int(a[x][y])-64]))
{
b[int(a[x][y])-64]=1;
search(x,y,k+1);
b[int(a[x][y])-64]=0;
}

}

}
int main()
{
scanf("%d%d",&r,&s);
for(int i=1;i<=r;i++)
for(int j=1;j<=s;j++)
cin>>a[i][j];
b[int(a[1][1])-64]=1;
search(1,1,1);
cout<<max1;
}