import sys
import math
#from collections import deque, defaultdict, Counter
#import heapq
#import bisect
#import itertools
#import functools

# 外部ライブラリ（AtCoder環境で利用可能）
#from sortedcontainers import SortedList, SortedSet, SortedDict
#from atcoder.dsu import DSU
#from atcoder.segtree import SegTree
#from atcoder.lazysegtree import LazySegTree
#from atcoder.fenwicktree import FenwickTree

# 入力高速化
#input = sys.stdin.readline
# 再帰回数上限
sys.setrecursionlimit(10**9)

# よくあるmod
#MOD = 1000000007
MOD = 998244353

def bubble_sort(L):
    action = []
    goal = list(sorted(L))
    for i, n in enumerate(goal):
        cur = L.index(n)
        while i != cur:
            action.append((cur-1, cur))
            L[cur-1], L[cur] = L[cur], L[cur-1]
            cur -= 1

    return action
def solve():
    # 解答ここから
    import heapq
    N, K = map(int, input().split(' '))
    A = list(map(int, input().split(' ')))
    sp = [[] for _ in range(K)]
    for i in range(N):
        if A[i] % K != (i+1) % K:
            print(-1)
            return
        sp[(i+1) % K].append(A[i])
    
    answer = []
    for k in range(1, K+1):
        actions = bubble_sort(sp[k % K])
        for i, j in actions:
            answer.append((K*i+k, K*j+k))
    print(len(answer))
    for i, j in answer:
        print(i, j)
if __name__ == '__main__':
    T = 1
    for _ in range(T):
        solve()