#include <bits/stdc++.h>
using namespace std;

const int MXN = 1000005;
int N, M, hei[MXN], stk[MXN], top(-1), lb[MXN], rb[MXN], ans;
char str[MXN];

inline int calc(int l, int r) {
  if (r > l) swap(l, r);
  static map<pair<int, int>, int> dp;
  if (!l && !r) return 0;
  auto it(dp.find(make_pair(l, r)));
  if (it != dp.end()) return it->second;
  int ret(1 + min(l ? calc(l / 2, (r + 1) / 2) : N * M,
                  r ? calc((l + 1) / 2, r / 2) : N * M));
//cout << l << ' ' << r << ' ' << ret << endl;
  return dp[make_pair(l, r)] = ret;
}

int main() {
  cin >> N >> M, ans = N * M;
  for (int i(0); i != N; ++i) {
    cin >> str;
    for (int j(0); j != M; ++j) {
      hei[j] = (str[j] == '1' ? hei[j] + 1 : 0);
      while (~top && hei[stk[top]] > hei[j]) rb[stk[top--]] = j;
      lb[j] = (~top ? stk[top] : -1);
      stk[++top] = j;
    }
    while (~top) rb[stk[top--]] = M;
    for (int j(0); j != M; ++j)
      if (hei[j]) {
        int l1(rb[j] - lb[j] - 1), l2(hei[j]);
        int ls1(lb[j] + 1), rs1(M - rb[j]);
        int ls2(i + 1 - hei[j]), rs2(N - 1 - i);
        ans = min(ans, calc((ls1 + l1 - 1) / l1, (rs1 + l1 - 1) / l1) +
                       calc((ls2 + l2 - 1) / l2, (rs2 + l2 - 1) / l2));
      }
  }
  cout << ans << endl;
  return 0;
}