int ktnguyento(int n)
{
	int dem = 0;
	for (int i = 1; i <= n; i++)
	{
		if (n % i == 0)
			dem++;
	}
	if (dem == 2)
		return 1;
	else
		return 0;
}

int songuyento(int n)
{
	if (n == 1)
		return n;
	else
	for (int i = n - 1; i > 1; i--)
	if (ktnguyento(i) == 1)
		return i;
}