#include <stdio.h>
#include <algorithm>

using namespace std;

/*int n , m;
vector < int > a[1200000] , b[1200000] , c[1200000];
char z[1200000];
int k;
int mx , ans;
int ok ( int x ) {
	if ( x == (x&(-x)) ) return 1;
	return 0;
}
void work () {
	int i , j , s;
	scanf ( "%d%d" , &n , &m );
	a[0].resize ( m + 1 );
	b[0].resize ( m + 1 );
	c[0].resize ( m + 1 );
	for ( i = 1 ; i <= n ; i++ ) {
		for ( j = 1 ; j <= m ; j++ ) {
			//z[j] = '0' + (rand () % 1000==0?1:0);
			if ( i == 1 && j == 1 ) z[j] = '1';
			else z[j] = '0';
		}
		//printf ( "%s\n" , z + 1 );
		//scanf ( "%s" , z + 1 );
		a[i].resize ( m + 1 );
		b[i].resize ( m + 1 );
		c[i].resize ( m + 1 );
		for ( j = 1 ; j <= m ; j++ ) {
			b[i][j] = a[i][j] = z[j] - '0';
			if ( z[j] == '1' ) {
				mx = max ( mx , max (n-i+1,m-j+1) );
			}
		}
	}
	while ( 1 ) {
		k++;
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				c[i][j] = (c[i-1][j] + c[i][j-1] + c[i-1][j-1] + b[i][j]) % 2;
				if ( ok(k) ) printf ( "%d " , c[i][j] );
			}
			if ( ok(k) ) printf ( "\n" );
		}
		if ( ok(k) ) printf ( "\n" );
		s = 0;
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				if ( a[i][j] == c[i][j] ) {
					s++;
				}
				b[i][j] = c[i][j];
			}
		}
		if ( s == n * m ) break;
	}
	printf ( "%d\n" , k );
	ans = 1;
	while ( ans < mx ) {
		ans = ans * 2;
	}
	printf ( "%d\n" , ans );
}*/

int n , m;
vector < int > a[1200000] , b[1200000] , c[1200000];
char z[1200000];
int k;
void work () {
	int i , j , l , o , s , s1 , s2 ,s3;
	//n = 1000000; m = 1;
	scanf ( "%d%d" , &n , &m );
	a[0].resize ( m + 1 );
	b[0].resize ( m + 1 );
	c[0].resize ( m + 1 );
	for ( i = 1 ; i <= n ; i++ ) {
		/*for ( j = 1 ; j <= m ; j++ ) {
			z[j] = '0' + (rand () % 2==0?1:0);
			//if ( i == 1 && j == 1 ) z[j] = '1';
			//else z[j] = '0';
		}*/
		scanf ( "%s" , z + 1 );
		a[i].resize ( m + 1 );
		b[i].resize ( m + 1 );
		c[i].resize ( m + 1 );
		for ( j = 1 ; j <= m ; j++ ) {
			b[i][j] = a[i][j] = z[j] - '0';
		}
	}
	k = 1;
	while ( 1 ) {
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				if ( i - k >= 1 ) s1 = c[i-k][j];
				else s1 = 0;
				if ( j - k >= 1 ) s2 = c[i][j-k];
				else s2 = 0;
				if ( i-k >= 1 && j-k >= 1 ) s3 = c[i-k][j-k];
				else s3 = 0;
				c[i][j] = (s1+s2+s3+a[i][j]) % 2;
			}
		}
		s = 0;
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				if ( a[i][j] == c[i][j] ) {
					s++;
				}
				b[i][j] = c[i][j];
			}
		}
		if ( s == n * m ) {
			//printf ( "%d\n" , k );
			//return ;
			break;
		}
		k = k * 2;
	}
	/*k = 128;
	while ( 1 ) {
		printf ( "%d\n" , k );
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				b[i][j] = 0;
			}
		}
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				if ( a[i][j] ) {
					for ( l = i ; l <= n ; l += k ) {
						for ( o = j ; o <= m ; o += k ) {
							b[l][o] ^= 1;
						}
					}
				}
			}
		}
		s = 0;
		for ( i = 1 ; i <= n ; i++ ) {
			for ( j = 1 ; j <= m ; j++ ) {
				if ( a[i][j] == b[i][j] ) {
					s++;
				}
				//printf ( "%d " , b[i][j] );
			}
			//printf ( "\n" );
		}
		//printf ( "\n" );
		if ( s == n * m ) break;
		k = k * 2;
	}*/
	printf ( "%d\n" , k );
}
int main () {
	work ();
	return 0;
}