n,m = map(int,raw_input().split())
if not 2 <= n <= 100000: exit()
if not n%2 == 0: exit()
if not 1 <= m <= n/2: exit()
x = input(); a = []
if not 1 <= x <= 1000000000: exit()
def f(d):
	b = a[:d]+a[d+1:]; c = 0
	i1 = 0; i2 = len(b)-1
	while(i1<i2):
		if b[i1]+b[i2]>a[d]+x:
			c+=1; i1+=1; i2-=1;
		else: i1+=1;
	return 1 if c < m else 0
for i in xrange(n-1):
	a.append(input())
for i in a:
	if not 1 <= i <= 1000000000: exit()
a.sort()
low = 0; high = len(a)-1
while(low < high):
	mid = (high+low)/2
	if f(mid) == 1: high = mid
	else: low = mid+1
print a[low] if f(low) == 1 else -1