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

int main(){
  int H, W;
  cin >> H >> W;
  assert(1 <= H <= 500);
  assert(1 <= W <= 500);
  vector<string> A(H + 2);
  for(int i = 1; i <= H; i++){
    cin >> A[i];
    assert(A[i].size() == W);
    for(auto c : A[i]){
      assert(c == '0' || c == '1');
    }
  }
  A[0] = string(W, '0');
  A[H + 1] = string(W, '1');
  sort(A.begin(), A.end());

  vector<int>dp(H + 2, 1e9);
  dp[0] = 0;
  for(int i = 0; i < H + 2; i++){
    for(int j = i + 1; j < H + 2; j++){
      bool ok = true;
      for(int k = 0; k < W; k++){
        if(A[i][k] == '1' && A[j][k] == '0'){
          ok = false;
          break;
        }
      }
      if(ok){
        dp[j] = min(dp[j], dp[i] + (j - i - 1));
      }
    }
  }
  cout << dp[H + 1] << endl;
}