import sys, math
sys.setrecursionlimit(1000000)
INF = 1 << 100
#mod = 1000000007
mod = 998244353
input = lambda: sys.stdin.readline().rstrip()
li = lambda: list(map(int, input().split()))

N, K = li()
if N // 2 + N % 2 > K:
    print(-1)
    exit()

out = []
for i in range(N - K):
    idx = (i*2) + 1
    out.append((idx, idx+1))

for i in range(N):
    out.append((i+1, i+1))

print(len(out))
for o in out:
    print(*o)