submatrix sum max

#include <bits/stdc++.h>
using namespace std;
int a[51][51],s[51][51];

int main()
{

    int n,m,i,j,maxi=-32000;
    cin>>n>>m;
    for (i=1; i<=n; i++)
        for (j=1; j<=m; j++)
            cin>>a[i][j];
    for (i=1; i<=n; i++)
        for (j=1; j<=m; j++)
            s[i][j]=a[i][j]+s[i][j-1]+s[i-1][j]-s[i-1][j-1];
    int x1,x2,y1,y2;
    for (x1=1; x1<=n; x1++)
        for (y1=1; y1<=m; y1++)
            for (x2=x1; x2<=n; x2++)
                for (y2=y1; y2<=m; y2++)
                    if (maxi<s[x2][y2]-s[x1-1][y2]-s[x2][y1-1]+s[x1-1][y1-1])
                        maxi=s[x2][y2]-s[x1-1][y2]-s[x2][y1-1]+s[x1-1][y1-1];

    cout<<maxi;
    return 0;
}