n,k = map(int,input().split())
if k < n - k :
    print(-1)
    exit()

edges = []
for i in range(k):
    edges.append([i+1,i+1])

for i in range(n-k):
    edges.append([k+1+i,i+1])
    edges.append([i+1,k+1+i])

print(len(edges))
for a,b in edges:
    print(a,b)