func validSquare(p1 []int, p2 []int, p3 []int, p4 []int) bool {
	seg := make(map[int]int)
	seg[dist2(p1, p2)]++
	seg[dist2(p1, p3)]++
	seg[dist2(p1, p4)]++
	seg[dist2(p2, p3)]++
	seg[dist2(p2, p4)]++
	seg[dist2(p3, p4)]++
	if len(seg) != 2 {
		return false
	}
	for _, v := range seg {
		if v&1 == 1 {
			return false
		}
	}
	return true
}

func dist2(p1, p2 []int) int {
	dx, dy := p1[0]-p2[0], p1[1]-p2[1]
	return dx*dx + dy*dy
}