#include <bits/stdc++.h>
#define ff first
#define ss second
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

void get(int l, int r, vector<pii> &V) {
    V.push_back({l, r});
    if (l == r) return;

    int m = (l + r) / 2;
    if ((r - l + 1) % 2 == 0) {
        get(l, m, V);
        get(m + 1, r, V);
    }
    else {
        get(l, m, V);
        get(m, r, V);
    }
}

int lg(int n) {
    return n == 1 ? 0 : __lg(n - 1) + 1;
}

int main() {
    cin.tie(0); ios_base::sync_with_stdio(0);
    
    int n, m;
    cin >> n >> m;
    int a[n + 1][m + 1]{};
    for (int i = 1; i <= n; i++) {
        string S;
        cin >> S;
        for (int j = 1; j <= m; j++) {
            a[i][j] = (S[j - 1] == '1');
        }
    }
    for (int i = 1; i <= n; i++) for (int j = 1; j <= m; j++) a[i][j] += a[i][j - 1];
    for (int i = 1; i <= n; i++) for (int j = 1; j <= m; j++) a[i][j] += a[i - 1][j];

    vector<int> X, Y;
    for (int x = n; ; x = (x + 1) / 2) {
        X.push_back(x);
        if (x == 1) break;
    }
    for (int y = m; ; y = (y + 1) / 2) {
        Y.push_back(y);
        if (y == 1) break;
    }

    bool gx[20][n + 1]{};
    bool gy[20][m + 1]{};
    gx[0][1] = 1;
    for (int i = 0; i < (int)X.size() - 1; i++) {
        for (int j = 1; j <= n - X[i] + 1; j++) if (gx[i][j]) {
            int sz = X[i];
            if (sz % 2 == 0) {
                gx[i + 1][j] = true;
                gx[i + 1][j + sz / 2] = true;
            }
            else {
                if (j > 1) gx[i + 1][j - 1] = true;
                gx[i + 1][j] = true;
                gx[i + 1][j + sz / 2] = true;
                if (j + sz / 2 + 1 <= n - X[i + 1] + 1) gx[i + 1][j + sz / 2 + 1] = true;
            }
        }
    }
    gy[0][1] = 1;
    for (int i = 0; i < (int)Y.size() - 1; i++) {
        for (int j = 1; j <= m - Y[i] + 1; j++) if (gy[i][j]) {
            int sz = Y[i];
            if (sz % 2 == 0) {
                gy[i + 1][j] = true;
                gy[i + 1][j + sz / 2] = true;
            }
            else {
                if (j > 1) gy[i + 1][j - 1] = true;
                gy[i + 1][j] = true;
                gy[i + 1][j + sz / 2] = true;
                if (j + sz / 2 + 1 <= m - Y[i + 1] + 1) gy[i + 1][j + sz / 2 + 1] = true;
            }
        }
    }

    vector<pii> XX, YY;
    for (int i = 0; i < X.size(); i++) for (int j = 1; j <= n - X[i] + 1; j++) {
        if (gx[i][j]) XX.push_back({j, j + X[i] - 1});
    }
    for (int i = 0; i < Y.size(); i++) for (int j = 1; j <= m - Y[i] + 1; j++) {
        if (gy[i][j]) {
            YY.push_back({j, j + Y[i] - 1});
        }
    }
    
    int ans = 1e9;
    for (pii p : XX) for (pii q : YY) {
        int cnt = a[p.ss][q.ss] - a[p.ff - 1][q.ss] - a[p.ss][q.ff - 1] + a[p.ff - 1][q.ff - 1];
        if (cnt == (p.ss - p.ff + 1) * (q.ss - q.ff + 1)) {
            ans = min(ans, lg(n) - lg(p.ss - p.ff + 1) + lg(m) - lg(q.ss - q.ff + 1));
        }
    }
    cout << ans << '\n';
}