#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


constexpr int MAX = 1'000'010;
constexpr int E = 20;

char buf[MAX];

int M, N;
vector<string> A;

// [l, dp[dir][e][l]) is enough to expand fully in e operations
int dp[2][E + 1][MAX];

int calc(int dir, int l, int r) {
  int lo = -1, hi = E;
  for (; lo + 1 < hi; ) {
    const int mid = (lo + hi) / 2;
    ((dp[dir][mid][l] <= r) ? hi : lo) = mid;
  }
  return hi;
}

int ans;
void check(int x0, int x1, int y0, int y1) {
  int cost = 0;
  cost += calc(0, x0, x1);
  cost += calc(1, y0, y1);
// cerr<<"[check] "<<x0<<" "<<x1<<" "<<y0<<" "<<y1<<": "<<cost<<endl;
  chmin(ans, cost);
}

int main() {
  for (; ~scanf("%d%d", &M, &N); ) {
    A.resize(M);
    for (int x = 0; x < M; ++x) {
      scanf("%s", buf);
      A[x] = buf;
    }
    
    for (int dir = 0; dir < 2; ++dir) {
      const int m = (dir == 0) ? M : N;
      for (int e = 0; e <= E; ++e) {
        fill(dp[dir][e], dp[dir][e] + (m + 1), m + 1);
      }
      dp[dir][0][0] = m;
      for (int e = 1; e <= E; ++e) {
        for (int l = 0, r = l + 1; l <= m; ++l) {
          for (; r <= m && !(
            dp[dir][e - 1][max(2 * l - r, 0)] <= r ||
            dp[dir][e - 1][l] <= min(2 * r - l, m)
          ); ++r) {}
          dp[dir][e][l] = r;
        }
// cerr<<dir<<" "<<e<<": ";pv(dp[dir][e],dp[dir][e]+(m+1));
      }
    }
    
    ans = 2 * E + 1;
    vector<int> hs(N + 1, 0);
    vector<int> stackY(N + 2), stackH(N + 2);
    for (int x = 0; x < M; ++x) {
      for (int y = 0; y < N; ++y) hs[y] = (A[x][y] == '1') ? (hs[y] + 1) : 0;
      int top = 0;
      stackY[top] = 0;
      stackH[top] = 0;
      for (int y = 0; y <= N; ++y) {
        stackY[top + 1] = y;
        for (; stackH[top] > hs[y]; --top) {
          check(x + 1 - stackH[top], x + 1, stackY[top], y);
        }
        stackH[++top] = hs[y];
      }
    }
    printf("%d\n", ans);
  }
  return 0;
}