P1255 数楼梯 60分

求助！

#include<bits/stdc++.h>
using namespace std;
int main()
{
    long long a[10005] = {1, 1}, n;
    cin >> n;
    int i = 2;
    while(i <= n)
	{
        a[i] = a[i - 1] + a[i - 2];
        i++;
    }
    cout << a[n];
    return 0;
}