#include<bits/stdc++.h>

using namespace std;
typedef unsigned int uint;
typedef long long int ll;
typedef unsigned long long int ull;

#define debugv(v) printf("L%d %s => ",__LINE__,#v);for(auto e:v){cout<<e<<" ";}cout<<endl;
#define debugm(m) printf("L%d %s is..\n",__LINE__,#m);for(auto v:m){for(auto e:v){cout<<e<<" ";}cout<<endl;}
#define debuga(m,w) printf("L%d %s is => ",__LINE__,#m);for(int x=0;x<(w);x++){cout<<(m)[x]<<" ";}cout<<endl;
#define debugaa(m,w,h) printf("L%d %s is..\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){cout<<(m)[x][y]<<" ";}cout<<endl;}
#define ALL(v) (v).begin(),(v).end()
#define BIGINT 0x7FFFFFFF
#define E107 1000000007
void printbit(int u){if(u==0)cout<<0;else{int s=0,k=0;for(;0<u;u>>=1,k++)s=(s<<1)|(u&1);for(;0<k--;s>>=1)cout<<(s&1);}}

#define TIME chrono::system_clock::now()
#define MILLISEC(t) (chrono::duration_cast<chrono::milliseconds>(t).count())

template<typename T1,typename T2>
ostream& operator <<(ostream &o,const pair<T1,T2> p){o<<"("<<p.first<<":"<<p.second<<")";return o;}

int width,height;
int m,n;

map<vector<vector<int>>,int> dp;

int main(){
    int i,j,k;
    int x,y,a,b;
    
    cin >> height >> width;
    
    vector<vector<int>> first; // valarray
    vector<vector<int>> final;
    
    for (y=0;y<height;y++){
        first.emplace_back(width);
        final.emplace_back(width);
        for (x=0;x<width;x++){
            cin >> first[y][x];
        }
    }
    
    // all zero
    
    queue<pair<int,vector<vector<int>>> > q;
    
    q.push(make_pair(0,final));
    
    while (!q.empty()){
        pair<int,vector<vector<int>>> p = q.front();
        q.pop();
        
        if (p.second == first){
            cout << p.first << endl;
            return 0;
        }
        
        for (y=1;y<height-1;y++){
            for (x=1;x<width-1;x++){
                vector<vector<int>> tmp = p.second;
                for (i=-1;i<=1;i++){
                    for (j=-1;j<=1;j++){
                        tmp[y+j][x+i]^=1;
                    }
                }
                int &val = dp[tmp];
                if (!val){
                    val = 1;
                    q.push(make_pair(p.first+1,tmp));
                }
            }
        }
        
        
    }
    
    cout << -1 << endl;

    return 0;
}