清晰明了

#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;

const long long INF = -1e18; // 更小的初始值
int n, m;
int a[1001][1001];
long long dp[1001][1001][3]; // 改用long long

int main() {
    cin >> n >> m;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> a[i][j];
            dp[i][j][0] = dp[i][j][1] = dp[i][j][2] = INF;
        }
    }
    // 初始化起点
    dp[0][0][0] = dp[0][0][1] = dp[0][0][2] = a[0][0];
    for (int j = 0; j < m; j++) {
        for (int i = 0; i < n; i++) {
            if (j > 0) {
                long long max_prev = max(dp[i][j-1][0], max(dp[i][j-1][1], dp[i][j-1][2]));
                if (max_prev != INF) {
                    dp[i][j][0] = max_prev + a[i][j];
                }
            }
            if (i > 0) {
                if (dp[i-1][j][1] != INF) {
                    dp[i][j][1] = max(dp[i][j][1], dp[i-1][j][1] + a[i][j]);
                }
                if (dp[i-1][j][0] != INF) {
                    dp[i][j][1] = max(dp[i][j][1], dp[i-1][j][0] + a[i][j]);
                }
            }
        }
        for (int i = n-2; i >= 0; i--) {
            if (dp[i+1][j][2] != INF) {
                dp[i][j][2] = max(dp[i][j][2], dp[i+1][j][2] + a[i][j]);
            }
            if (dp[i+1][j][0] != INF) {
                dp[i][j][2] = max(dp[i][j][2], dp[i+1][j][0] + a[i][j]);
            }
        }
    }
    long long result = max(dp[n-1][m-1][0], max(dp[n-1][m-1][1], dp[n-1][m-1][2]));
    cout << (result == INF ? -1 : result) << endl; // 处理不可达
    return 0;
}