def solve(N,K):
    if N>2*K:
        return None

    E=[]
    for i in range(1,N-K+1):
        p=2*i-1; q=2*i
        E.append((p,p))
        E.append((p,q))
        E.append((q,p))

    for i in range(2*(N-K)+1,N+1):
        E.append((i,i))
    return E
#==================================================
N,K=map(int,input().split())
E=solve(N,K)

if E==None:
    print(-1)
else:
    print(len(E))
    for a,b in E:
        print(a,b)