n,k = map(int,input().split())
if k == 0 or k < n//2:
    print(-1)
    exit()
ans = [[i,i] for i in range(1,n+1)]

sans = []
k = n-k
while k:
    a,b = ans.pop()
    c,d = ans.pop()
    sans.append([a,d])
    sans.append([a,b])
    sans.append([c,d])
    k -= 1

while ans:
    sans.append(ans.pop())
print(len(sans))
for i in sans:
    print(*i)