#include<cstdio>
using namespace std;
int n,m,in,jn,maxn,cnt;
char a[12][12];
bool b[140];
void s(int istep,int jstep){
	if(istep == m + 1 || jstep == n + 1 || b[a[jstep - jn][istep - in]]){
		if(maxn < cnt) maxn = cnt;
		cnt = 0;
		return ;
	}
	for(int j = -1;j <= 1;j += 2){
		for(int i = -1;i <= 1;i += 2){
			in = i;
			jn = j;
			b[a[istep + i][jstep + j]] = 1;
			cnt++;
			s(istep + i,jstep + j);
			b[a[istep + i][jstep + j]] = 0;
		}
	}
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i = 1;i <= m;i++){
		for(int j = 1;j <= n;j++){
			scanf("%c",&a[i][j]);
		}
	}
	s(1,1);
	printf("%d",maxn);
	return 0;
}