#include <bits/stdc++.h>

using namespace std;
int main(){
    long long rows, cols;
    cin >> rows >> cols;

    long long sum = 0;
    long long min = -1;
    for(long long i = 0; i < rows; i++){
        for(long long j = 0; j < cols; j++){
            long long x;
            cin >> x;
            if(rows % 2 == 0 && cols % 2 == 0){
                if(!(i == j || rows-i == cols-j)){
                    if(min == -1 || x < min) {
                        min = x;
                    }
                }
            }
            sum += x;
        }
    }

    if (min != -1)
        sum -= min;
    cout << sum << endl;
}