# 2491. Divide Players Into Teams of Equal Skill

Medium
You are given a positive integer array `skill` of even length `n` where `skill[i]` denotes the skill of the `ith` player. Divide the players into `n / 2` teams of size `2` such that the total skill of each team is equal.
The chemistry of a team is equal to the product of the skills of the players on that team.
Return the sum of the chemistry of all the teams, or return `-1` if there is no way to divide the players into teams such that the total skill of each team is equal.
Example 1:
Input: skill = [3,2,5,1,3,4]
Output: 22
Explanation:
Divide the players into the following teams: (1, 5), (2, 4), (3, 3), where each team has a total skill of 6.
The sum of the chemistry of all the teams is: 1 * 5 + 2 * 4 + 3 * 3 = 5 + 8 + 9 = 22.
Example 2:
Input: skill = [3,4]
Output: 12
Explanation:
The two players form a team with a total skill of 7.
The chemistry of the team is 3 * 4 = 12.
Example 3:
Input: skill = [1,1,2,3]
Output: -1
Explanation:
There is no way to divide the players into teams such that the total skill of each team is equal.
Constraints:
• `2 <= skill.length <= 10^5`
• `skill.length` is even.
• `1 <= skill[i] <= 1000`

### 解題

Runtime: 82 ms, faster than 90.27%
Memory Usage: 8.4 MB, less than 80.53%
func dividePlayers(skill []int) int64 {
// 兩個兩個一組
sort.Ints(skill)
ans := 0
l := len(skill)
sum := skill + skill[l - 1]
for i:=0; i <= (l/2) - 1; i++ {
if skill[i] + skill[l - i - 1] != sum { return int64(-1) }
ans += skill[i] * skill[l - i - 1]
}
return int64(ans)
}