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 = 0, 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; } } 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) { } }