int binsearch()
{
	int l = 0, r = min(a, b) * (n - 1), mid = 0;
	while (r - l > 1)
	{
		mid = l + (r - l) / 2;
		if (mid/a + mid/b <= n-1)
			l = mid;
		else
			r = mid;
	}
	return l;
}