Maximum Sum

#include<iostream>
#include<cstdio>
#include<cstring>

using namespace std;
const int maxn = 1005;
int dp[ maxn ][ maxn ], num[ maxn ][ maxn ];

int main(){
	int n;
	while( cin >> n ){
		memset( num, 0, sizeof( num ) );
		for( int i = 1; i <= n; ++i ){
			for( int j = 1; j <= n; ++j ){
				cin >> num[ i ][ j ];
				num[ i ][ j ] += num[ i - 1 ][ j ];
			}
		}
		int sum = 0;
		int Max = num[ 1 ][ 1 ];
		for( int i = 1; i <= n; ++i ){
			for( int j = i; j <= n; ++j ){
				int sum = 0;
				for( int k = 1; k <= n; ++k ){
					if( sum < 0 ){
						sum = num[ j ][ k ] - num[ i ][ k ];
					}
					else if( i != j ){
						sum += num[ j ][ k ] - num[ i ][ k ];
					}
					if( sum > Max )
						Max = sum;
				}
			}
		}
		cout << Max << endl;
	}
	return 0;
}