#include<bits/stdc++.h>
#define int long long
using namespace std;
int n,m;
string a[1000010],b[1000010];
bool check(int d){
	for(int i = 0; i < n; i++) b[i] = a[i];
	for(int i = 0; i < n; i++){
		for(int j = 0; j < m; j++){
			int tmp = b[i][j] - '0';
			if(i >= d) tmp ^= (b[i - d][j] - '0');
			if(j >= d) tmp ^= (b[i][j - d] - '0');
			if(i >= d && j >= d) tmp ^= (b[i - d][j - d] - '0');
			b[i][j] = tmp + '0';
			if(a[i][j] != b[i][j]) return 0;
		}
	}
	return 1;
}
signed main(){
	scanf("%lld %lld",&n,&m);
	for(int i = 0; i < n; i++) cin>>a[i];
	for(int k = 0; ; k++){
		if(check(1LL << k)){
			printf("%lld",(1LL << k));
			return 0;
		}
	}
	return 0;
}