Efficient algorithm for 486A of Codeforces

fn.cpp

#include <iostream>
 
using namespace std;
 
typedef long long int lli;
 
int main() {
	lli n{0};
	cin >> n;
	if (n == 3) {
		cout << -1;
	}
	else if (n % 2 == 0) {
		cout << n / 2;
	}
	else if (n % 2) {
		auto out = (((n / 2) + 1) * (-1));
		cout << out;
	}
	return 0;
 
}