题解 | 长椅安置

#include <iostream>
using namespace std;

typedef long long ll;

//计算m * (m - 1) * ... *(m - n + 1)  
ll factorial(ll m, ll n) {
	ll result = 1;
	while (n-- > 0) {
		result *= m;
		m--;
	}
	return result;
}

int main() {
    ll n, ans = 1;
    cin >> n;
	ll a = factorial(n, 5);
	ll b = factorial(5, 5);
  	//这个()要注意，不然a * a会超出long long
	ans = a * (a / b); 
	printf("%lld", ans);
}