Secret Santa

input/code.cpp: In function 'int main()':
input/code.cpp:38:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < n; i++) v.push_back(i);
                      ^
input/code.cpp:43:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < n; i++) {
                       ^

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef unsigned long long int ull;
typedef long double ld;

int main() {
	ull n;
	cin >> n;
	
	ull u = 0;
	ull d = 1;
	
	cout.precision(20);
	cout<<fixed;
	
	if (n > 20) {
		cout << 0.63212055882855767839 << endl;
		return 0;
	}
	
	for (ull i = 2; i <= n; i++) {
		u *= i;
		if (i % 2 == 0) u++;
		else u--;
		d *= i;
		cout.precision(20);
		cout<<fixed;
		//cout<<u<<" / "<<d<<" -> "<<1-ld(u)/d<<endl;
	}
	cout<<1-ld(u)/d<<endl;
	return 0;
	
	vector<int> v;
	for (int i = 0; i < n; i++) v.push_back(i);
	
	int res = 0;
	
	do {
		for (int i = 0; i < n; i++) {
			if (v[i] == i) goto err;
		}
		res += 1;
		err:;
	} while (next_permutation(v.begin(), v.end()));
	
	cout << res << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: ACCEPTED