import sys
#input = sys.stdin.readline
input = sys.stdin.buffer.readline #文字列はダメ
#sys.setrecursionlimit(1000000)
#import bisect
#import itertools
#import random
#from heapq import heapify, heappop, heappush
#from collections import defaultdict 
#from collections import deque
#import copy
#import math
#from functools import lru_cache
#@lru_cache(maxsize=None)
MOD = pow(10,9) + 7
#MOD = 998244353
#dx = [1,0,-1,0]
#dy = [0,1,0,-1]
#dx8 = [1,1,0,-1,-1,-1,0,1]
#dy8 = [0,1,1,1,0,-1,-1,-1]


def main():
    N,K = map(int,input().split())
    if N == K:
        print(N)
        for i in range(1,N+1):
            ret = [i,i]
            print(*ret)
        exit()
    if N%2 == 0:
        if K < N//2:
            print(-1)
            exit()
    else:
        if K <= N//2:
            print(-1)
            exit()

    x = N-1-K
    ans = []
    #print("x",x)
    for i in range(x+1):
        ret = [2*i+1,2*i+2]
        ans.append(ret)
        # print(*ret)
    l = 2*x + 2; r = 2*x+3
    while True:
        if r >= N: break
        ret = [l,r]
        ans.append(ret)
        # print(*ret)
        l += 1; r += 1
    
    for i in range(1,N+1):
        ret = [i,i]
        ans.append(ret)
        # print(*ret)
    
    print(len(ans))
    for ret in ans:
        print(*ret)



if __name__ == '__main__':
    main()