玄关

#include <iostream>
#include <algorithm>

using namespace std;

int n, m;
int mp[1001][1001];
int face[1001][1001][2]; //只有下，右两个方向（因为向上或向左就走远了）

long long LNF = 0x3f3f3f3f3f3f3f3f;

int dfs(int x, int y, /*坐标*/ int from /*从哪个方向来的*/)
{
	if(x < 1 || x > n || y < 1 || y > m)
		return 0;
	if(face[x][y][from] != LNF)
		return face[x][y][from]; //如果搜过这里，返回搜到的数据	
	if(from == 0)
		face[x][y][from] = max({dfs(x + 1, y, 0), dfs(x, y - 1, 0), dfs(x, y - 1, 1)}) + mp[x][y];
	else
		face[x][y][from] = max({dfs(x - 1, y, 1), dfs(x, y - 1, 0), dfs(x, y - 1, 1)}) + mp[x][y];
	
	return face[x][y][from];
	
}

signed main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);// 关闭同步流
	
	cin >> n >> m;
	for(int i = 1; i <= n; i++)
	{
		for(int j = 1; j <= m; j++)
		{
			cin >> mp[i][j];
			face[i][j][0] = -LNF;
			face[i][j][1] = -LNF;
		}
	}
	
	face[1][1][0] = face[1][1][1] = mp[1][1];
	cout << dfs(n, m, 1) << endl;
	
	return 0;
}