#include<bits/stdc++.h>
using namespace std;
int n,m,a[1001][1001],up[1001][1001],down[1001][1001],dp[1001][1001];//up从下至上,down从上至下
int main()
{
 	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin >> n >> m;
	for(int i = 1;i <= n;i++) for(int j = 1;j <= m;j++) cin >> a[i][j];
	for(int i = 1;i <= n;i++) for(int j = 1;j <= m;j++){
		up[i][j] = max(up[i + 1][j],dp[i][j - 1]) + a[i][j];
		down[i][j] = max(down[i - 1][j],dp[i][j - 1]) + a[i][j];
		dp[i][j] = max(up[i][j],down[i][j]);//获取最终最大值
	}
	cout << dp[n][m];
    return 0;
}

在线等，急！！！谢谢orz