问题标题: 酷町堂：酷町堂5071，WA10分，啊啊啊啊啊啊啊啊啊求解！！！！

My code

    #include <iostream>
    #include <algorithm>
    #include <cstdio>
    #include <cmath>
    #include <string>
    #include <cstring>
    #include <map>
    #include <vector>
    #include <queue>
    #include <stack>
    #define MAXN 1000010
    typedef long long ll;
    using namespace std;
    int read()
    {
        int ans = 0;
        char ch = getchar();
        while (!isdigit(ch) && ch != '-') ch = getchar();
        bool sign = false;
        if (ch == '-') sign = true, ch = getchar();
        while (ch >= '0' && ch <= '9') ans = ans * 10 + (ch - '0'), ch = getchar();
        return (sign == true ? -ans : ans);
    }
    #define F(a) for(int i=1;i<=a;i++)
    long long test,a[MAXN];
    long long f(int n,int x)
    {
        if(a[n]) return a[n];
        if(n==1) return 1;
        long long int ans=0;
        for(int i=x;i<=3&&i<=n;i++)
        {
            ans+=f(n-i,i);
        }
        a[n]=ans;
        return ans;
    }

    int main()
    {
        int n;
        n=read();
        cout<<f(n,1);
        return 0;
    }

但是只有十分，请Dalao帮忙康康啊啊啊！