import sys

n, k = [int(i) for i in input().split()]
a = list(map(int, input().split()))

b = sorted(a)

location_a = {}
for i in range(n):
    location_a[b[i]] = i

ans = 0

for i in range(k):
    l = []
    j = i
    while j < n:
        l.append(j)
        j += k
    
    sz = len(l)
    
    visited = [False]*sz
    
    for j in range(sz):
        if visited[j]:
            continue
        
        c = 0
        d = j
        
        while not visited[d]:
            visited[d] = True
            location_b = location_a[a[l[d]]]
            
            if location_b % k != i:
                print(-1)
                sys.exit()
            
            d = (location_b - i)//k
            c += 1
        
        if c > 1:
            ans += c - 1

print(ans)