import sys
input = sys.stdin.readline

N,K=map(int,input().split())

if K<=N//2:
    print(-1)
else:
    x=N-K

    ANS=[]

    for i in range(1,N+1):
        if i+x<=N:
            ANS.append((i,i+x))

    if x!=0:
        for i in range(1,N+1):
            ANS.append((i,i))

    print(len(ANS))
    for x,y in ANS:
        print(x,y)