import sys
input = sys.stdin.readline

N,K=map(int,input().split())

if K==0:
    print(-1)
    exit()

edge = [(1,1)]
for i in range(2,N+1):
    if K<N:
        edge.append((i-1,i))
        K+=1
    edge.append((i,i))

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