import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
n, k = na()
s = n//2

if n - k <= s:
    print(n + k)
    for i in range(k):
        print(i*2+1,i*2+2)
    for i in range(n):
        print(i+1,i+1)


else:
    print(-1)