#include<bits/stdc++.h>
using namespace std;

int n, m, a0;
vector<int> a;

bool check(int x){
    int k = a0 + x;
    int cnt = 0;
    int left = 0, right = a.size() - 1;
    while(right - left > 0){
        int point = a[right] + a[left];
        if(point > k){
            cnt++;
            right--;
            left++;
        }else{
            left++;
        }
    }
    return cnt < m;
}

int main(){
    cin >> n >> m >> a0;
    for(int i=0;i<n-1;++i){
        int tmp;
        cin >> tmp;
        a.push_back(tmp);
    }
    sort(a.begin(), a.end());
    int ng = -1, ok = a.size() - 1;
    while(ok - ng > 1){
        int mid = (ok + ng) / 2;
        if(check(mid))ok = mid;
        else ng = mid;
    }

    if(!check(ok))cout << -1;
    else cout << ok;
    cout << endl;

    return 0;
}