[실5] 14916 - 거스름돈

#include <iostream>

using namespace std;

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);

	int n, res = 0; cin >> n;

	if (n < 5)
		res = (n % 2 == 0) ? n / 2 : -1;

	else
	{
		int q = n / 5;

		if (n % 5 == 0)
			res = q;

		else
		{
			int remain = n - q * 5;

			if (remain % 2 == 0)
				res = q + remain / 2;

			else
				res = (q - 1) + (remain + 5) / 2;
		}
	}
	cout << res;
	return 0;
}