N, M = list(map(int, input().split(' ')))

A = []
for i in range(N):
    A.append(int(input()))

K = A[0]    
A = sorted(A, reverse=True)
used = [False for _ in range(len(A))]
done = 0

for i in range(len(A)-2):
    for j in reversed(range(len(A))):
        if not used[j]:
            x = A[i] + A[j]
            y = A[i+1] + A[i+2]
            if y <= x:
                if (A[i] == K or A[j] == K) and done < M:
                    print(x - K)
                    exit(0)
                else:
                    done = done + 1
                    used[j] = True

print(-1)