#include <bits/stdc++.h>
using namespace std;
int arr[2505][2505], sum[2505][2505], n, m;
int get_sum(int x1, int y1, int x2, int y2) {
  x2 = min(x2, n);
  y2 = min(y2, m);
  assert(x1 <= x2);
  assert(y1 <= y2);
  return sum[x2][y2] - sum[x2][y1 - 1] - sum[x1 - 1][y2] + sum[x1 - 1][y1 - 1];
}
int main() {
  ios::sync_with_stdio(false);
  ;
  cin.tie(0);
  ;
  int i, j, k, ans = 100000000, ones, zeroes;
  string s;
  cin >> n >> m;
  for (i = 1; i <= n; i++) {
    cin >> s;
    s = " " + s;
    for (j = 1; j <= m; j++) arr[i][j] = s[j] - '0';
  }
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= m; j++)
      sum[i][j] = sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1] + arr[i][j];
  }
  for (k = 2; k <= max(n, m); k++) {
    int got = 0;
    for (i = 1; i <= n; i = i + k) {
      for (j = 1; j <= m; j = j + k) {
        ones = get_sum(i, j, i + k - 1, j + k - 1);
        zeroes = k * k - ones;
        got += min(ones, zeroes);
      }
    }
    ans = min(ans, got);
  }
  cout << ans;
  return 0;
}
