求助！！第8个TLE

#include<iostream>
#include<cstdio>
#include<string>
using namespace std;
int a[1010][1010],b[1010][1010]={0},n;
int fi(int x,int y)
{
	if (x==n)
		return a[x][y];
	if (b[x][y]!=0)
		return b[x][y];
	int c,m;
	m=fi(x+1,y);
	c=fi(x+1,y+1);
	b[x][y]=max(c,m)+a[x][y];
	return b[x][y];
}
int main()
{
	int m,i,j;
	cin>>n;
	for (i=1;i<=n;i++)
		for (j=1;j<=i;j++)
			cin>>a[i][j];
    int f=1;
	m=fi(1,1);
	cout<<m;
	return 0;
}