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_a = []
    group_b = []
    
    j = i
    while j < n:
        group_a.append(a[j])
        group_b.append(b[j])
        j += k
    
    if sorted(group_a) != sorted(group_b):
        print(-1)
        sys.exit()
    
    # đếm số swap tối thiểu
    pos = {v: idx for idx, v in enumerate(group_b)}
    
    visited = [False] * len(group_a)
    
    for j in range(len(group_a)):
        if visited[j] or pos[group_a[j]] == j:
            continue
        
        cycle = 0
        x = j
        
        while not visited[x]:
            visited[x] = True
            x = pos[group_a[x]]
            cycle += 1
        
        if cycle > 1:
            ans += cycle - 1

print(ans)