调了一个上午。

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

int n, m;
int a[1010][1010];
int f[1010][1010][5];
int INF = numeric_limits<int>::min();

int dfs(int i, int j, int k) {
	if (1 > i || i > n) return 0;
	if (1 > j || j > m) return 0;
	if (f[i][j][k] != INF) return f[i][j][k];
	if (k == 0) {
		f[i][j][0] = max(max(dfs(i, j - 1, 0), dfs(i, j - 1, 1)), dfs(i, j - 1, 2)) + a[i][j];
	}
	if (k == 1) {
		f[i][j][1] = max(dfs(i - 1, j, 0), dfs(i - 1, j, 1)) + a[i][j];
	}
	if (k == 2) {
		f[i][j][2] = max(dfs(i + 1, j, 0), dfs(i + 1, j, 2)) + a[i][j];
	}
	return f[i][j][k];
}

int main() {
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			cin >> a[i][j];
			f[i][j][0] = f[i][j][1] = f[i][j][2] = INF;
		}
	}
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			for (int k = 0; k < 3; k++) {
				dfs(i, j, k);
			}
		}
	}
	cout << max(max(f[n][m][0], f[n][m][1]), f[n][m][2]) << endl;
	return 0;
}