from atcoder.fenwicktree import FenwickTree
N, K = map(int, input().split())
A = list(map(int, input().split()))

arrs = [[] for _ in range(K)]
for i, a in enumerate(A):
    if a%K != (i+1)%K:
        print(-1)
        exit()
    arrs[i%K].append([a, i])

ans = []
def rotate(A):
    ft = FenwickTree(N+1)
    for a, i in A:
        ft.add(a, 1)
        ct = ft.sum(a+1, N+1)
        for c in range(ct):
            ans.append([i-K*(c+1) , i-K*c])

for arr in arrs: 
    rotate(arr)
print(len(ans))
# print(ans)
for i, j in ans:
    print(f"{i+1} {j+1}")