import sys
n, m = map(int, input().split())
a0 = int(input())
a = [int(input()) for x in range(n - 1)]
a.sort()
#-1 check
l = 0
r = n - 3
cnt = 0
while l < r:
	while l < r and a[l] + a[r] <= a[n - 2] + a0:
		l += 1
	if l < r:
		l += 1
		r -= 1
		cnt += 1
if cnt >= m:
	print(-1)
	sys.exit(0)
low = 0
high = n - 2
for i in range(50):
	mid = int((low + high) / 2)
	l = 0
	r = n - 2
	cnt=0
	while l < mid and mid < r:
		while l < mid and a[l] + a[r] <= a[mid] + a0:
			l += 1
		if l<mid:
			l += 1
			r -= 1
			cnt += 1
	while mid + 1 < r and a[r-1]+a[r]>a[mid]+a0:
		r -= 2
		cnt += 1
	while l+1<mid and a[l]+a[l+1]>a[mid]+a0:
		l+=2
		cnt+=1
	if cnt >= m:
		low = mid
	else:
		high = mid
print(a[high])