90分求调！

#include<iostream>
#include<algorithm>
using namespace std;
int sum[1005][1005], a[1005][1005], f[1005][1005], n, m;
int main()
{
	cin >> n >> m;
	for (int i = 1; i <= n; i++)
		for (int j = 1; j <= m; j++) {
			cin >> a[i][j];
			sum[i][j] = sum[i - 1][j] + a[i][j];
		}
	f[1][1] = a[1][1];
	for (int i = 2; i <= n; i++)
		f[i][1] = f[i - 1][1] + a[i][1];
	for (int j = 2; j <= m; j++) {
		for (int i = 1; i <= n; i++) {
			f[i][j] = f[i][j - 1] + a[i][j];
		}
		int val = f[1][j - 1];
		for (int i = 2; i <= n; i++) {
			val = max(val, f[i - 1][j - 1] - sum[i - 2][j]);
			f[i][j] = max(f[i][j], val + sum[i][j]);
		}
		val = f[n][j - 1] + sum[n][j];
		for (int i = n - 1; i >= 1; i--) {
			val = max(val, f[i + 1][j - 1] + sum[i + 1][j]);
			f[i][j] = max(f[i][j], val - sum[i - 1][j]);
		}
	}
	cout << f[n][m];
	return 0;
}