#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#define N 1017
using namespace std;
int n,m;
char s[N][N];
int main()
{
	cin >> n >> m;
	for(int i = 1;i <= n;i += 1)cin >> s[i]+1;
	int ans = 0;
	for(int i = 2;i <= n;i += 1){
		int x = strlen(s[i-1]+1),y = strlen(s[i]+1);
		int len = 0;
		while(s[i-1][x] == s[i][y] && x >= 1 && y >= 1)x--,y--,len++;
		ans = max(ans,len);
	}
	cout << ans << endl;
	return 0;
}