《洛谷提高—的题难度只有9》

#include<bits/stdc++.h>
using namespace std;
//#define ii int
//#define ll long long
//#define cc char
//#define dd double
//#define in cin
//#define out cout
//#define pp printf
//简化
/*bool sushu(int x){
if(x<2){
return false;
}
if(x==2||x==3){
return true;//TODO
}
for(int i=2;i*i<=x;i++){
if(x%i==0)return false;
}return true;
}
// 素数*/
int main()
{	
	unsigned long long a,ans;cin>>a;
	ans=a*(a-1)/2*(a-2)/3*(a-3)/4;
		if(a<4){
			cout<<0;//TODO
		}else cout<<ans;//n⋅(n−1)/2⋅(n−2)/3⋅(n−3)/4
	return 0;
}