前缀和二维
#include <iostream>
#include<algorithm>
using namespace std;

const int N = 505;

int n, m, q;
int s[N][N];

int main()
{
    scanf("%d%d", &n, &m);

    for (int i = 1; i <= n; i ++ )
        for (int j = 1; j <= m; j ++ )
            scanf("%d", &s[i][j]);

    for (int i = 1; i <= n; i ++ )
        for (int j = 1; j <= m; j ++ )
            s[i][j] += s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1];
    int ans=0;
   //printf("%d\n", s[x2][y2] - s[x1 - 1][y2] - s[x2][y1 - 1] + s[x1 - 1][y1 - 1]);
    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=m;j++)
        {
            for(int k=1;k<=n;k++)
            {
                for(int l=1;l<=m;l++)
                {
                    int x1=i,y1=j,x2=k,y2=l;
                    int temp=s[x2][y2] - s[x1 - 1][y2] - s[x2][y1 - 1] + s[x1 - 1][y1 - 1];
                    ans=max(ans,temp);
                }
            }
        }
    }
    cout<<ans;
    return 0;
}