import sys

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

b = sorted(a)

groups = [[] for _ in range(k)]

# chia nhóm
for i in range(n):
    groups[i % k].append(a[i])

# sort từng nhóm
for i in range(k):
    groups[i].sort()

# ghép lại
new_a = [0]*n
idx = [0]*k

for i in range(n):
    g = i % k
    new_a[i] = groups[g][idx[g]]
    idx[g] += 1

# kiểm tra khả thi
if new_a != b:
    print(-1)
    sys.exit()

# đếm swap từng nhóm
ans = 0

for i in range(k):
    arr = groups[i]
    
    # đếm swap tối thiểu để sort arr (cycle method)
    visited = [False]*len(arr)
    pos = sorted(range(len(arr)), key=lambda x: arr[x])
    
    for j in range(len(arr)):
        if visited[j] or pos[j] == j:
            continue
        
        cycle = 0
        x = j
        
        while not visited[x]:
            visited[x] = True
            x = pos[x]
            cycle += 1
        
        if cycle > 1:
            ans += cycle - 1

print(ans)