import sys
input = sys.stdin.readline

def judge(x):
    p = a0+a[x]
    b = [a[i] for i in range(N-1) if i != x]
    pair = 0
    l, r = 0, N-3
    
    while l <= r:
        if b[l]+b[r] >= p+1:
            pair += 1
            r -= 1
        
        l += 1
    
    return pair <= M-1
    
def binary_search():
    left, right = 0, N-2
    
    while left <= right:
        mid = (left+right)//2
        
        if judge(mid):
            right = mid-1
        else:
            left = mid+1
    
    return left

N, M = map(int, input().split())
a0 = int(input())
a = [int(input()) for _ in range(N-1)]
a.sort()

idx = binary_search()

if idx == N-1:
    print(-1)
else:
    print(a[idx])