#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 1e6 + 5;
int n, m, ans;
vector<int> a[N], b[N];
vector<ll> vec1, vec2;
int st[25][N];
int p1[N], p2[N];
int ask(int l1, int r1, int l2, int r2){
	return b[r1][r2] - b[l1 - 1][r2] - b[r1][l2 - 1] + b[l1 - 1][l2 - 1];
}
void build(int n){
	int log2n = log2(n);
	for (int i = 1; i <= n; i++) st[0][i] = p1[i];
	for (int j = 1; j <= log2n; j++)
		for (int i = 1; i + (1 << j) - 1 <= n; i++)
			st[j][i] = min(st[j - 1][i], st[j - 1][i + (1 << (j - 1))]);
}
int getmin(int l, int r){
	int t = 31 ^ __builtin_clz(r - l + 1);
	return min(st[t][l], st[t][r - (1 << t) + 1]);
}
bool check2(int l, int r){
	return getmin(l, r) <= r;
}
bool check(int n, int l, int r){
	int len = r - l + 1;
	return check2(max(1, l - len), r) || check2(l, min(r + len, n));
}
ll tr1(int l, int r, int d){
	return l + 1ll * r * N + 1ll * d * N * N;
}
int main(){
	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
//	freopen("in.txt", "r", stdin);
//	freopen("out.txt", "w", stdout);
	cin >> n >> m;
	a[0] = b[0] = vector<int>(m + 1, 0);
	for (int i = 1; i <= n; i++){
		a[i] = b[i] = vector<int>(m + 1, 0);
		for (int j = 1; j <= m; j++){
			char c;
			cin >> c;
			a[i][j] = c - '0';
			b[i][j] = b[i][j - 1] + a[i][j];
		}
	}
	for (int i = 1; i <= n; i++)
		for (int j = 1; j <= m; j++)
			b[i][j] += b[i - 1][j];
	for (int i = 1; i <= n; i++) p1[i] = i == 1 ? n : n + 1;
	vec1.push_back(tr1(1, n, 0));
	for (int cur = 1; ; cur++){
		build(n);
		bool flg = 1;
		for (int i = 1; i <= n; i++){
			p2[i] = p1[i];
			if (i != p1[i]) flg = 0;
		}
		if (flg) break;
		int p = n;
		for (int i = n; i >= 1; i--){
			while (p >= i && check(n, i, p)) p--;
			p1[i] = p + 1;
			if (i <= p1[i] && p1[i] <= n) vec1.push_back(tr1(i, p1[i], cur));
		}
	}
	for (int i = 1; i <= m; i++) p1[i] = i == 1 ? m : m + 1;
	vec2.push_back(tr1(1, m, 0));
	for (int cur = 1; ; cur++){
		build(m);
		bool flg = 1;
		for (int i = 1; i <= m; i++){
			p2[i] = p1[i];
			if (i != p1[i]) flg = 0;
		}
		if (flg) break;
		int p = m;
		for (int i = m; i >= 1; i--){
			while (p >= i && check(m, i, p)) p--;
			p1[i] = p + 1;
			if (i <= p1[i] && p1[i] <= m) vec2.push_back(tr1(i, p1[i], cur));
		}
	}
	ans = 1e9;
	for (ll v1 : vec1)
		for (ll v2 : vec2){
			int l1 = v1 % N, r1 = (v1 / N) % N, d1 = v1 / N / N;
			int l2 = v2 % N, r2 = (v2 / N) % N, d2 = v2 / N / N;
			if (ask(l1, r1, l2, r2) == (r1 - l1 + 1) * (r2 - l2 + 1))
				ans = min(d1 + d2, ans);
		}
	cout << ans;
	return 0;
}