import sys
input=lambda: sys.stdin.readline().rstrip()
n,k=map(int,input().split())
A=[int(i) for i in input().split()]
AA=sorted([(i,a) for i,a in enumerate(A)],key=lambda x:x[1])
D=dict()
for i,(ind,aa) in enumerate(AA):
  D[ind]=i

chk=True
R=[[] for _ in range(k)]
ans=0
for i,a in enumerate(A):
  if (i-D[i])%k!=0:
    chk=False
    break
  else:
    for r in R[i%k]:
      if r>a:
        ans+=1
    R[i%k].append(a)
print(ans if chk else -1)