import std.conv, std.functional, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.complex, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int N, M;
int[] A;

int solve(int I) {
  int ret;
  for (int l = 1, r = N - 1; r >= 0; --r) {
    if (r != I) {
      for (; l < r && !(l != I && A[l] + A[r] > A[0] + A[I]); ++l) {}
      if (l >= r) {
        break;
      }
      ++ret;
      ++l;
    }
  }
  debug {
    writefln("I = %s: ret = %s", I, ret);
  }
  return ret;
}

void main() {
  try {
    for (; ; ) {
      N = readInt();
      M = readInt();
      A = new int[N];
      foreach (i; 0 .. N) {
        A[i] = readInt();
      }
      
      A[1 .. N].sort;
      int lo = 0, hi = N;
      for (; lo + 1 < hi; ) {
        const mid = (lo + hi) / 2;
        (solve(mid) < M) ? (hi = mid) : (lo = mid);
      }
      writeln((hi == N) ? -1 : A[hi]);
    }
  } catch (EOFException e) {
  }
}