import sys
input = sys.stdin.readline

N, K = map(int, input().split())
if 2*K - N < 0:
    print(-1)
    exit()
U = 2*K - N
O = N - K
ans = []
for i in range(U+O):
    ans.append((i, i))
for i in range(O):
    ans.append((U+i, U+O+i))
    ans.append((U+O+i, U+i))
print(len(ans))
for a, b in ans:
    print(a+1, b+1)