import sys
from collections import defaultdict, deque

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

b = sorted(a)

# lưu tất cả vị trí của từng giá trị
pos = defaultdict(deque)
for i in range(n):
    pos[b[i]].append(i)

ans = 0

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

print(ans)