N, K = map(int, input().split())
A = list(map(int, input().split()))

for i in range(N):
    if (i+1)%K != A[i]%K:
        print(-1)
        exit()

ops = []
for k in range(K):
    for i in range(k, N, K):
        for j in range(i+K, N, K):
            if A[i] > A[j]:
                ops.append((i+1, j+1))
                A[i], A[j] = A[j], A[i]

print(len(ops))
for x in ops:
    print(*x)