Codeforces Round #677 (Div. 3) E. Two Round Dances 组合数学

E. Two Round Dances

input

20

output

12164510040883200

code

//Siberian Squirrel
#include<bits/stdc++.h>

#define IO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)
//#define ACM_LOCAL

using namespace std;
typedef long long ll;

const int N = 2e5 + 10;
const int MOD = 1e9 + 7;

int n, m;

ll quick_pow(ll ans, ll p, ll res = 1) {
    
    
	for(; p; p >>= 1, ans = ans * ans % MOD)
		if(p & 1) res = res * ans % MOD;
	return res % MOD;
}

ll inv(ll ans) {
    
    
	return quick_pow(ans, MOD - 2);
}

inline void solve(ll res = 1, ll ans = 1) {
    
    
	for(ll i = n / 2 + 1; i <= n; ++ i) {
    
    
		ans = ans * i;
	}
	for(ll i = 1; i <= n / 2; ++ i) {
    
    
		ans /= i;
	}
	ans = ans / 2;
	for(ll i = 1; i <= n / 2 - 1; ++ i) {
    
    
		res *= i;
	}
	cout << ans * res * res << endl;;
}

int main() {
    
    
	IO;
#ifdef ACM_LOCAL
	freopen("input", "r", stdin);
	freopen("output", "w", stdout);
#endif
	int o = 1;
//    cin >> o;
	while(o --) {
    
    
		cin >> n;
		solve();
	}
	return 0;
}