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 = []
    target = []
    
    j = i
    while j < n:
        group.append(a[j])
        target.append(b[j])
        j += k
    
    if sorted(group) != sorted(target):
        print(-1)
        sys.exit()
    
    group.sort()
    
    diff = 0
    for x, y in zip(group, target):
        if x != y:
            diff += 1
    
    ans += diff // 2

print(ans)