#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)



#define INF INT_MAX/2
int N, M, _A[101010], A0; int *A;
//-----------------------------------------------------------------------------------
bool chk(int x) {
    int idx = lower_bound(A, A + N, x) - A;

    int P = A0 + A[idx];
    int cnt = 0;
    int L = 0, R = N - 1;
    while (L < R) {
        if (L == idx) L++;
        else if (R == idx) R--;
        else if (A[L] + A[R] <= P) L++;
        else {
            cnt++;
            L++; R--;
        }
    }

    return cnt < M;
}
//-----------------------------------------------------------------------------------
int main() {
    cin >> N >> M;
    rep(i, 0, N) scanf("%d", &_A[i]);

    A0 = _A[0];
    A = _A + 1; N--;
    sort(A, A + N);

    int lo = 0, hi = A[N - 1] + 1;
    while (lo + 1 != hi) {
        int md = (lo + hi) / 2;
        if (chk(md)) hi = md;
        else lo = md;
    }

    if (hi == A[N - 1] + 1) hi = -1;
    printf("%d\n", hi);

    //cout << chk(1) << endl;
    //cout << chk(2) << endl;
}