import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
import time
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines
write=sys.stdout.write

N,K=map(int,readline().split())
if N>K*2:
    print(-1)
else:
    ans_lst=[]
    for i in range(K):
        ans_lst.append((i,N-1-i))
    for i in range(N):
        if i*2!=N-1:
            ans_lst.append((i,i))
    print(len(ans_lst))
    for a,b in ans_lst:
        print((a+1,b+1))