#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = l; i <= r; i++)
#define per(i, r, l) for (int i = r; i >= l; --i)
using namespace std;
typedef long long ll;
const int N = 1000006;
const int inf = 1e9;
int n, m;
string s[N];
string t[N];
bool check(int d) {
    rep(i, 0, n - 1) t[i] = s[i];
    rep(i, 0, n - 1) {
        rep(j, 0, m - 1) {
            bool now = t[i][j] - '0';
            if (i >= d) now ^= t[i - d][j] - '0';
            if (j >= d) now ^= t[i][j - d] - '0';
            if (i >= d && j >= d) now ^= t[i - d][j - d] - '0';
            t[i][j] = now + '0';
        }
    }
    rep(i, 0, n - 1) {
        if (t[i] != s[i]) return false;
    }
    return true;
}
int main() {
    scanf("%d%d", &n, &m);
    rep(i, 0, n - 1) { cin >> s[i]; }
    for (int k = 0; (1 << k) <= max(n, m) * 2; k++) {
        if (check(1 << k)) {
            printf("%d\n", 1 << k);
            break;
        }
    }
    return 0;
}