矩阵取数问题

#include <bits/stdc++.h>
#define _xx ios_base::sync_with_stdio(0);cin.tie(0);
using namespace std;
typedef long long ll;
ll dp[505][505] = {0};
ll a[505][505] = {0};
int main()
{
    ll n;
    cin >> n;
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= n; j++) cin >> a[i][j];
    for(int i = 0; i <= n; i++) dp[i][0] = dp[0][i] = 0;
    for(int i = 1; i <= n; i++)
    {
        for(int j = 1; j <= n; j++)
        {
            dp[i][j] = max(dp[i - 1][j], dp[i][j - 1]) + a[i][j];
        }
    }
    cout << dp[n][n] << endl;
    return 0;
}