N,K = map(int,input().split())
import sys
if N % 2 == 0 and K < N // 2:
    print(-1)
    exit()
if N % 2 == 1 and K <= N // 2:
    print(-1)
    exit()
    
ans = []
for i in range(1,K + 1):
    ans.append((i,i))
u = N - K
for i in range(1,u + 1):
    ans.append((i,K + i))
    ans.append((K + i,i))
print(len(ans))
for a,b in ans:
    print(a,b)