N, K = (int(x) for x in input().split())
P = list(map(int, input().split()))
ok = True
ans = 0
op = []
for i in range(K):
    p = []
    j = i+1
    while j <= N:
        p.append(P[j-1])
        j += K
    q = sorted(p.copy())
    for num, j in enumerate(q):
        if j != num*K+(i+1):
            ok = False
    for _ in range(len(p)):
        for j in range(len(p)-1):
            if p[j] >= p[j+1]:
                p[j], p[j+1] = p[j+1], p[j]
                ans += 1
                op.append([j*K+i+1, j*K+i+1+K])

if not ok:
    print(-1)
    exit()

print(ans)
for i, j in op:
    print(i, j)