import std.stdio, std.array, std.string, std.conv, std.algorithm; import std.typecons, std.range, std.random, std.math, std.container; import std.numeric, std.bigint, core.bitop, core.stdc.string; void main() { auto s = readln.split.map!(to!int); auto N = s[0]; auto M = s[1]; auto a0 = readln.chomp.to!int; auto A = (N-1).iota.map!(_ => readln.chomp.to!int).array; bool ok(int m) { int x = a0 + A[m]; int cnt = 0; auto rbt = new RedBlackTree!(int, "a < b", true)(); foreach (i; 0..N-1) if (i != m) rbt.insert(A[i]); foreach (i; 0..N-1) { if (i == m) continue; if (rbt.equalRange(A[i]).empty) continue; rbt.removeKey(A[i]); auto ub = rbt.upperBound(x - A[i]); if (ub.empty) continue; auto y = ub.front; rbt.removeKey(y); cnt += 1; } return cnt < M; } if (!ok(N-2)) { writeln(-1); return; } int hi = N-2; int lo = 0; while (hi - lo > 1) { int mid = (hi + lo) / 2; if (ok(mid)) hi = mid; else lo = mid; } writeln( ok(hi) ? A[hi] : A[lo]); }