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)