N, K = map(int, input().split())

A = list(map(int, input().split()))

B = A.copy()
B.sort()
ans = 0

for i in range(N):
    
    if A[i] != B[i]:
        
        x = A[i]
        for j in range(i + K, N, K):
            if A[j] == B[i]:
                A[j] = x
                A[i] = B[i]
                ans += 1
                break
            
            else:
                A[j], x = x, A[j]
                ans += 1
        
        if A[i] != B[i]:
            print(-1)
            exit()
            
print(ans)