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

int n, m, k, lb, ub;
std::vector<ll> vec;

bool check(int mid) {
    ll score = k + vec[mid];
    std::vector<ll> vec2;
    vec2 = vec;
    vec2[mid] = -1;
    std::vector<ll> points;
    for (int j = 1; j <= (vec2.size()-1)/2; j++) {
        points.push_back(vec2[j*2] + vec2[j*2-1]);
    }
    points.push_back(score);
    sort(points.begin(), points.end());
    auto it = upper_bound(points.begin(), points.end(), score);
    ll dis = distance(points.end(), it);
    if (dis < m) {
        return true;
    }
    return false;
}

int main() {
    bool exist_ans = false;
    vec.assign(n-1, 0);
    cin >> n >> m;
    cin >> k;

    for (int i = 0; i < n - 1; i++) {
        cin >> vec[i];
    }
    sort(vec.begin(), vec.end());
    lb = 0;
    ub = vec.size()-1;
    while (ub - lb > 1) {
        int mid = (ub + lb) / 2;
        if (check(mid)) {
            exist_ans = true;
            ub = mid;
        }
        else {
            lb = mid;
        }
    }
    if (exist_ans) {
        std::cout << vec[lb] + k << std::endl;
        return 0;
    }
    std::cout << -1 << std::endl;


}