import sys
sys.setrecursionlimit(10**7)

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

b = sorted(a)

ans = 0

for i in range(k):
    cur = []
    need = []
    
    j = i
    while j < n:
        cur.append(a[j])
        need.append(b[j])
        j += k
    
    if sorted(cur) != sorted(need):
        print(-1)
        sys.exit()
    
    # tạo mapping value -> list vị trí trong need
    from collections import defaultdict, deque
    
    pos = defaultdict(deque)
    for idx, val in enumerate(need):
        pos[val].append(idx)
    
    # xây permutation
    perm = []
    for val in cur:
        perm.append(pos[val].popleft())
    
    # đếm cycle
    visited = [False]*len(perm)
    
    for j in range(len(perm)):
        if visited[j]:
            continue
        
        cycle = 0
        x = j
        
        while not visited[x]:
            visited[x] = True
            x = perm[x]
            cycle += 1
        
        if cycle > 1:
            ans += cycle - 1

print(ans)