K=int(input().split()[1])
S=list(map(int,input().split()))
C=0
def s():
    global S,K
    c=0
    for i in range(len(S)-K):
        if S[i]>S[i+K]:
            S[i],S[i+K]=S[i+K],S[i]
            c+=1
    return c

while 1:
    c=s()
    C+=c
    if c<1:break
print(C if S==sorted(S) else -1)