#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e6+5,M=1005,inf=1e9;
int n,m,res=inf,dp[M][M],dy1[N],dy2[N];
vector<int>p[N],tmp[N];
bitset<N>q[M],nw;
set<int>jl;
multiset<int>rs;
map<int,int>f1[N],f2[N];
int lg2(int x){
	if(x==0)return -1;
	return __lg(x);
}
int gets(int l,int r,int n){
	if(l<1)l=1;
	if(r>n)r=n;
	if(l==1&&r==n)return 0;
	if(n==m){
		if(f2[l].count(r))return f2[l][r];
		return f2[l][r]=min(r<n?gets(l,l+2*(r-l)+1,n):inf,l>1?gets(r-2*(r-l)-1,r,n):inf)+1;
	}else{
		if(f1[l].count(r))return f1[l][r];
		return f1[l][r]=min(r<n?gets(l,l+2*(r-l)+1,n):inf,l>1?gets(r-2*(r-l)-1,r,n):inf)+1;
	}
}
void del(int x){
	auto w=jl.insert(x).first,w1=w,w2=w;
	w1--;w2++;
	int l=(*w1)+1,r=(*w2)-1;
	if(l<=r)rs.erase(rs.find(gets(l,r,m)));
	if(l<x)rs.insert(gets(l,x-1,m));
	if(x<r)rs.insert(gets(x+1,r,m));
}
int gets(){
	return *(rs.begin());
}
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		p[i].resize(m+1);
		for(int j=1;j<=m;j++){
			char c;
			cin>>c;
			p[i][j]=c-'0';
		}
	}
	if(n>m){
		swap(n,m);
		for(int i=1;i<=n;i++){
			tmp[i].resize(m+1);
			for(int j=1;j<=m;j++)tmp[i][j]=p[j][i];
		}
		for(int i=1;i<=m;i++)p[i].clear();
		for(int i=1;i<=n;i++)p[i]=tmp[i];
	}
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			q[i][j]=p[i][j]^1;
		}
	}
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++)nw[j]=1;
		jl.clear();rs.clear();
		jl.insert(0);jl.insert(m+1);
		rs.insert(0);
		for(int j=i;j<=n;j++){
			auto g=nw&q[j];
			for(int k=g._Find_first();k<N;k=g._Find_next(k)){
			//	cout<<k<<endl;
				del(k);
				nw[k]=0;
			}
			if(!rs.size())break;
			//cout<<i<<" "<<j<<" "<<n<<" "<<gets(i,j,n)<<" "<<gets()<<endl;
			res=min(res,gets(i,j,n)+gets());
			//cout<<i<<" "<<j<<" "<<res<<endl;
		}
	}
	cout<<res;
}