请大佬修理代码：

#include<bits/stdc++.h>
using namespace std;
unsigned long long n,a = 0,b = 1,c = 1,d;
int main(){
	cin>>n;
	for(long long i = 2;i<=n;i++){
		d = a + b + c;
		a = b,b = c,c = d;
	}
	cout<<d<<endl;
	return 0;
}
/*

*/