兄弟们最爱的动态规划入门第一题：

#include <bits/stdc++.h>
using namespace std;


long long a[75]={0};//不用longlong后面会爆掉

int main() {
	a[0]=1,a[1]=1,a[2]=2;

	for(int i=3;i<=71;i++){
		a[i] = a[i-1]+a[i-2]+a[i-3];
//		cout<<"i = "<<i<<" a[i] = "<<a[i]<<endl;checkpoint 
	}

	int n;
	while(cin>>n){
		cout<<a[n]<<endl;
		
	}	
    
    return 0;
}