题解

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

long long f[60];

int main() {

	f[1] = 1;
	f[2] = 2;
	f[3] = 4; 
	for(int i = 4; i <= 50; i++) {
		f[i] = f[i-1] + f[i-2] + f[i-3];
	}

	int t;
	cin >> t;
	cout << f[t] << endl; 

	return 0;
}