import sys
readline=sys.stdin.readline

def Bisect_Int(ok,ng,is_ok):
    while abs(ok-ng)>1:
        mid=(ok+ng)//2
        if is_ok(mid):
            ok=mid
        else:
            ng=mid
    return ok

N,M=map(int,readline().split())
a=int(readline())
A=[int(readline()) for i in range(N-1)]
A.sort()
def is_ok(i):
    if i==N-1:
        return True
    AA=[A[ii] for ii in range(N-1) if i!=ii]
    cnt=0
    l,r=0,N-2
    while l<r:
        while l+1<r and A[l]+A[r]<=a+A[i]:
            l+=1
        if l<r and A[l]+A[r]>a+A[i]:
            cnt+=1
        l+=1
        r-=1
    return cnt<M
ans=Bisect_Int(N-1,0,is_ok)
if ans==N-1:
    ans=-1
print(ans)