2491. Divide Players Into Teams of Equal Skill

class Solution {
public:
    long long dividePlayers(vector<int>& skill) {
        
        long long n = skill.size();
        sort(skill.begin() , skill.end());
        vector<long long>a;
        long long start = 0,end = n-1;
        long long sum = skill[0] + skill[n-1];
        long long k = 0;
        
        while(start<=end){
            if((skill[start] + skill[end])!= sum) return -1;
            a.push_back(skill[start]);
            a.push_back(skill[end]);
            start++;end--;k++;
        }
        k=0;
        long long ans = 0;
        for(long long i = 0 ; i < a.size() ; i+=2){
            ans+=a[i]*a[i+1];
            k++;
        }
        return ans;
    }
};