N,K = map(int,input().split())
if (N+1)//2 <= K:
    cnt = 0
    ANS = []
    for x in range(1,N+1):
        ANS.append((x,x))
    while cnt < K :
        for x in range(1,N+1):
            if x % 2 == 1:
                if x + 1 <= N:
                    ANS.append((x,x+1))
                    cnt += 1
    while cnt < K:
        for x in range(1,N+1):
            if x % 2 == 0:
                if x + 1 <= N:
                    ANS.append((x,x+1))
                    cnt += 1
    print(len(ANS))
    for x,y in ANS:
        print(x,y)
else:
    print(-1)