func arrayPairSum(nums []int) int {
	set := [20001]int{}
	for _, i := range nums {
		set[i+10000]++
	}
	sum, flag := 0, 1
	for i, val := range set {
		if val != 0 {
			if val&1 == 0 {
				sum += val / 2 * (i - 10000)
			} else {
				sum += (val + flag) / 2 * (i - 10000)
				flag ^= 1
			}
		}
	}
	return sum
}

func arrayPairSumSort(nums []int) int {
	sort.Ints(nums)
	sum := 0
	for i := len(nums) - 2; 0 <= i; i -= 2 {
		sum += nums[i]
	}
	return sum
}