#include<bits/stdc++.h>
#define ulg unsigned long long
#define lg long long
#define ui unsigned int
using namespace std;
const int N=505,H=1013;
inline int read(){
	int XX=0,FF=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-')
			FF*=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		XX=XX*10+ch-48;
		ch=getchar();
	}
	return XX*FF;
}
inline void write(int x){
	if(x<0){
		putchar('-');
		x=-x;
	}
	if(x>9)
		write(x/10);
	putchar(x%10+'0');
}
ulg n,m;
ulg has[2*N][N],p[N*2],ans=0;
map<ulg,bool>d;
bool check(int x){
	for(int i=0;i+x<m;i++){
		bool flag=1;
		d.clear();
		for(int j=1;j<=n;j++){
			d[has[j][i+x]-has[j][i]*p[x]]=1;
		}
		for(int j=n+1;j<=n*2;j++){
			if(d[has[j][i+x]-has[j][i]*p[x]]==1){
				flag=0;
				break;
			}
		}
		if(flag)return 1;
	}
	return 0;
}
int main(){
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
	n=read();
	m=read();
	p[0]=1;
	for(int i=1;i<=m;i++)p[i]=p[i-1]*H;
	for(int i=1;i<=n*2;i++){
		char c;
		for(int j=1;j<=m;j++){
			c=getchar();
			has[i][j]=has[i][j-1]*H+c;
		}
	}
	int l=1,r=m,mid;
	while(l<=r){
		mid=l+r>>1;
		if(check(mid)){
			ans=mid;
			r=mid-1;
		}
		else l=mid+1;
	}
	write(ans);
	return 0;
}

都帮朋友调好了我的还是不行。。。