import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.io.OutputStream; import java.io.PrintWriter; import java.math.BigDecimal; import java.util.AbstractMap; import java.util.ArrayDeque; import java.util.ArrayList; import java.util.Arrays; import java.util.Comparator; import java.util.Deque; import java.util.HashMap; import java.util.HashSet; import java.util.List; import java.util.Map; import java.util.Queue; import java.util.Set; import java.util.TreeSet; import static java.util.Comparator.*; public class Main { public static void main(String[] args) { InputStream inputStream = System.in; OutputStream outputStream = System.out; MyInput in = new MyInput(inputStream); PrintWriter out = new PrintWriter(outputStream); Solver solver = new Solver(); solver.solve(1, in, out); out.close(); } // ====================================================================== // static class Solver { int N,M; long[] A; // index が条件を満たすかどうか boolean isOK(long own, int pair) { long score = own + A[pair]; int lank = 1; for(int i=A.length-1; i >= 1; i-=2) { if(i == pair && (i-2) >= 0 && score < (A[i-1]+A[i-2])) { lank++; i--; } else if((i-1) == pair && (i-2) >= 0 && score < (A[i]+A[i-2])) { lank++; i--; } else if(score < (A[i]+A[i-1])) { lank++; } if(lank > M) break; } if (lank <= M) return true; else return false; } // 汎用的な二分探索のテンプレ // この場合、key より大きい中で 一番左のインデックスを返す int calc(long a) { int ng = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1 int ok = A.length; // 「index = a.length-1」が条件を満たさないこともあるので、初期値は a.length() /* ok と ng のどちらが大きいかわからないことを考慮 */ while (Math.abs(ok - ng) > 1) { int mid = (ok + ng) / 2; if (isOK(a, mid)) ok = mid; else ng = mid; } return ok; } public void solve(int testNumber, MyInput in, PrintWriter out) { N = in.nextInt(); M = in.nextInt(); A = new long[N-1]; long a = in.nextLong(); for(int i=0; i < N-1; i++) { A[i] = in.nextLong(); } Arrays.sort(A); long ans = calc(a); if(ans < N - 1) out.println(ans); else out.println(-1); } } // ====================================================================== static class Pair extends AbstractMap.SimpleEntry { /** serialVersionUID. */ private static final long serialVersionUID = 6411527075103472113L; public Pair(final K key, final V value) { super(key, value); } } static class MyInput { private final BufferedReader in; private static int pos; private static int readLen; private static final char[] buffer = new char[1024 * 8]; private static char[] str = new char[500 * 8 * 2]; private static boolean[] isDigit = new boolean[256]; private static boolean[] isSpace = new boolean[256]; private static boolean[] isLineSep = new boolean[256]; static { for (int i = 0; i < 10; i++) { isDigit['0' + i] = true; } isDigit['-'] = true; isSpace[' '] = isSpace['\r'] = isSpace['\n'] = isSpace['\t'] = true; isLineSep['\r'] = isLineSep['\n'] = true; } public MyInput(InputStream is) { in = new BufferedReader(new InputStreamReader(is)); } public int read() { if (pos >= readLen) { pos = 0; try { readLen = in.read(buffer); } catch (IOException e) { throw new RuntimeException(); } if (readLen <= 0) { throw new MyInput.EndOfFileRuntimeException(); } } return buffer[pos++]; } public int nextInt() { int len = 0; str[len++] = nextChar(); len = reads(len, isSpace); int i = 0; int ret = 0; if (str[0] == '-') { i = 1; } for (; i < len; i++) ret = ret * 10 + str[i] - '0'; if (str[0] == '-') { ret = -ret; } return ret; } public long nextLong() { int len = 0; str[len++] = nextChar(); len = reads(len, isSpace); int i = 0; long ret = 0L; if (str[0] == '-') { i = 1; } for (; i < len; i++) ret = ret * 10 + str[i] - '0'; if (str[0] == '-') { ret = -ret; } return ret; } public String nextString() { String ret = new String(nextDChar()).trim(); return ret; } public char[] nextDChar() { int len = 0; len = reads(len, isSpace); char[] ret = new char[len + 1]; for (int i=0; i < len; i++) ret[i] = str[i]; ret[len] = 0x00; return ret; } public char nextChar() { while (true) { final int c = read(); if (!isSpace[c]) { return (char) c; } } } int reads(int len, boolean[] accept) { try { while (true) { final int c = read(); if (accept[c]) { break; } if (str.length == len) { char[] rep = new char[str.length * 3 / 2]; System.arraycopy(str, 0, rep, 0, str.length); str = rep; } str[len++] = (char) c; } } catch (MyInput.EndOfFileRuntimeException e) { } return len; } static class EndOfFileRuntimeException extends RuntimeException { } } }