// Author: winterfrost (TLX account: Todoroki_Shouto)
// Contest: Singapore National Olympiad in Informatics 2014
// Problem: A. Orchard
# include <bits/stdc++.h>
using namespace std;
# define GRIND ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int main() {
GRIND
// For subtasks with small constraints
int n, m; cin >> n >> m;
char tree[n + 1][m + 1];
int alex = 0, bert = 0;
int prefAlex[n + 1][m + 1];
int prefBert[n + 1][m + 1];
memset(prefAlex, 0, sizeof(prefAlex));
memset(prefBert, 0, sizeof(prefBert));
for(int i = 1; i <= n; i++) {
for(int j = 1; j <= m; j++) {
cin >> tree[i][j];
if(tree[i][j] == '0') alex++;
else bert++;
prefAlex[i][j] = prefAlex[i - 1][j] + prefAlex[i][j - 1] - prefAlex[i - 1][j - 1] + (tree[i][j] == '0' ? 1 : 0);
prefBert[i][j] = prefBert[i - 1][j] + prefBert[i][j - 1] - prefBert[i - 1][j - 1] + (tree[i][j] == '1' ? 1 : 0);
}
}
int ans = INT_MAX;
for(int i = 1; i <= n; i++) {
for(int j = 1; j <= m; j++) {
for(int k = i; k <= n; k++) {
for(int l = j; l <= m; l++) {
int apples = prefAlex[k][l] - prefAlex[i - 1][l] - prefAlex[k][j - 1] + prefAlex[i - 1][j - 1];
int bananas = prefBert[k][l] - prefBert[i - 1][l] - prefBert[k][j - 1] + prefBert[i - 1][j - 1];
int cost = (bert - bananas) + apples;
ans = min(ans, cost);
}
}
}
}
cout << ans << endl;
return 0;
}
