下面放上代码

#include<bits/stdc++.h>
using namespace std;
int main(){
	int num;
	cin>>num;
	if(num==3){
		cout<<"0";return 0;
	}
	unsigned long long lines=0;
	lines=num*(num-1)*(num-2)*(num-3)/24;
	printf("%lld",lines);
	return 0;
}

十分感谢各位大佬