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

int main()
{
  int H, W;
  cin >> H >> W;
  assert(1 <= H && H <= 22);
  assert(1 <= W && W <= 22);

  vector S(H, vector(W, 0));
  for (int i = 0; i < H; i++)
  {
    for (int j = 0; j < W; j++)
    {
      cin >> S[i][j];
      assert(S[i][j] == 0 || S[i][j] == 1);
    }
  }

  vector dp(H + 1, vector(W + 1, 0));
  for (int i = H - 1; i >= 0; i--)
  {
    for (int j = W - 1; j >= 0; j--)
    {
      vector<int> row(H + 1, 0);
      vector<int> col(W + 1, 0);
      vector ng(H + 1, vector(W + 1, false));
      vector count(H + 1, vector(W + 1, 0));

      for (int x = H - 1; x >= i; x--)
      {
        for (int y = W - 1; y >= j; y--)
        {
          if (S[x][y] or ng[x + 1][y] or ng[x][y + 1])
          {
            ng[x][y] = true;
          }
          if (S[x][y])
          {
            count[x + 1][y + 1]++;
            row[x + 1] = 1;
            col[y + 1] = 1;
          }
        }
      }
      for (int x = 0; x < H; x++)
      {
        row[x + 1] += row[x];
      }
      for (int y = 0; y < W; y++)
      {
        col[y + 1] += col[y];
      }
      int mi = 1e9;
      for (int x = i + 1; x <= H; x++)
      {
        for (int y = j + 1; y <= W; y++)

        {
          count[x][y] += count[x - 1][y] + count[x][y - 1] - count[x - 1][y - 1];
          if (!ng[x][y])
          {
            mi = min(mi, dp[i][y] + dp[x][j] + row[x] * col[y] - count[x][y]);
          }
        }
      }
      dp[i][j] = mi;
    }
  }
  cout << dp[0][0] << endl;
  return 0;
}