#include <bits/stdc++.h>
#define ii pair<int, int>
#define fi first
#define se second
#define inf 0x3f3f3f3f
using namespace std;
using ll = long long;
const ll mod=1e9+7;
int n, m, ans=inf;
vector<vector<int>> s0, s1;
vector<vector<char>> a;
int get(int x, int y, int u, int v, int k)
{
if(k==0) return s0[u][v]-s0[x-1][v]-s0[u][y-1]+s0[x-1][y-1];
return s1[u][v]-s1[x-1][v]-s1[u][y-1]+s1[x-1][y-1];
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin>>n>>m;
a.assign(n+2, vector<char>(m+2));
s0.assign(n+2, vector<int>(m+2, 0));
s1.assign(n+2, vector<int>(m+2, 0));
for(int i = 1; i <= n; i++)
{
for(int j = 1; j <= m; j++)
{
cin>>a[i][j];
s0[i][j]=s0[i-1][j]+s0[i][j-1]-s0[i-1][j-1]+(a[i][j]=='0');
s1[i][j]=s1[i-1][j]+s1[i][j-1]-s1[i-1][j-1]+(a[i][j]=='1');
}
}
ans=min(ans, min(s0[n][m], s1[n][m]));
for(int l = 1; l <= n; l++)
{
for(int r = l; r <= n; r++)
{
int base[2];
for(int j = 0; j < 2; j++)
base[j]=get(1, 1, l-1, m, j)+get(r+1, 1, n, m, j);
int one=0, zero=0;
for(int i = 1; i <= m; i++)
{
int cur[2];
for(int j = 0; j < 2; j++)
cur[j]=get(l, 1, r, i, j);
// if(l==2 && r==4 && i==6) cout<<cur[0]<<' '<<cur[1]<<' '<<one<<'\n';
ans=min(ans, cur[0]-one+base[1]+get(l, i+1, r, m, 1));
one=max(one, cur[0]-cur[1]);
zero=max(zero, cur[1]-cur[0]);
}
}
}
cout<<ans;
}
