题解

#include <bits/stdc++.h>
using namespace std;
long long a[31];
int main(){
	int n;
	cin>>n;
    a[1] = 1;
    a[2] = 2;
    a[3] = 4;
    a[4] = 7;
    a[5] = 12; // 前面打个表
	for (int i=5;i<=n;i++) 
    {
        a[i] = a[i-1]+a[i-2]+a[i-4];
    }
	cout<<a[n];
	return 0;
}