import sys

n, k = map(int, input().split())
a = list(map(int, input().split()))

b = sorted(a)

ans = 0

for i in range(k):
    group = []
    j = i
    while j < n:
        group.append(a[j])
        j += k
    
    target = []
    j = i
    while j < n:
        target.append(b[j])
        j += k
    
    if sorted(group) != sorted(target):
        print(-1)
        sys.exit()
    
    # chuẩn bị để đếm swap
    arr = list(enumerate(group))
    arr.sort(key=lambda x: x[1])
    
    visited = [False]*len(arr)
    
    for j in range(len(arr)):
        if visited[j] or arr[j][0] == j:
            continue
        
        cycle = 0
        x = j
        
        while not visited[x]:
            visited[x] = True
            x = arr[x][0]
            cycle += 1
        
        if cycle > 1:
            ans += cycle - 1

print(ans)