#!/usr/bin/env python
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip

import random
import collections
import math
import itertools
import bisect

def real_main():
    n, k = read_int_array()
    max_pairs = (n - 2) * (n - 1) // 2
    if k > max_pairs:
        print(-1)
        return

    cur_pairs = max_pairs
    edges = []
    for i in range(1, n):
        edges.append([1, i+1])
    cur_node = 1
    last = [i for i in range(n)]
    for _ in range(cur_pairs - k):
        if last[cur_node] == n-1:
            cur_node += 1
        last[cur_node] += 1
        edges.append([cur_node + 1, last[cur_node] + 1])
    print(len(edges))
    print('\n'.join(str(e[0]) + ' ' + str(e[1]) for e in edges))


def solve():
    pass


def main():
    if False and 'PYCHARM_HOSTED' in os.environ:
        main_pycharm()
    else:
        real_main()


from copy import deepcopy
def main_pycharm():
    solution = solve

    test_inputs = None
    test_outputs = None
    judge = None
    slow_solution = None
    if solution is not None:
        if test_outputs is not None:
            test_with_answers(solution, test_inputs, test_outputs)
        if judge is not None:
            test_with_judge(solution, test_inputs, judge)
        if slow_solution is not None:
            test_with_slow_solution(solution, test_inputs, slow_solution)


def test_with_answers(solution, inputs, answers):
    total, wrong = 0, 0
    for args, test_ans in zip(inputs, answers):
        ans = solution(*deepcopy(args))
        if ans != test_ans:
            print('WRONG! ans=%s, test_ans=%s, args=%s' % (ans, test_ans, args))
            wrong += 1
        else:
            print('GOOD')
        total += 1
    print('ALL %d TESTS PASSED' % total if not wrong else '%d out of %d tests are WRONG' % (wrong, total))


def test_with_judge(solution, inputs_gen, judge):
    total, wrong = 0, 0
    for args in inputs_gen:
        ans = solution(*deepcopy(args))
        if not judge(deepcopy(ans), *deepcopy(args)):
            print('WRONG! ans=%s, args=%s' % (ans, args))
            wrong += 1
        total += 1
    print('ALL %d TESTS PASSED' % total if not wrong else '%d out of %d tests are WRONG' % (wrong, total))


def test_with_slow_solution(solution, inputs_gen, solution_slow):
    total, wrong = 0, 0
    for args in inputs_gen:
        ans = solution(*deepcopy(args))
        slow = solution_slow(*deepcopy(args))
        if ans != slow:
            print('WRONG! ans=%s, slow=%s, args=%s' % (ans, slow, args))
            wrong += 1
        total += 1
    print('ALL %d TESTS PASSED' % total if not wrong else '%d out of %d tests are WRONG' % (wrong, total))


# This mergesort can be like 7 times faster than build in sort
# (for stupid reasons)
def mergesort(A, key=lambda x: x, reverse=False):
    C = A
    A = list(range(len(A)))
    B = list(A)

    n = len(A)
    for i in range(0, n - 1, 2):
        if key(C[A[i]]) > key(C[A[i ^ 1]]):
            A[i], A[i ^ 1] = A[i ^ 1], A[i]

    width = 2
    while width < n:
        for i in range(0, n, 2 * width):
            R1, R2 = min(i + width, n), min(i + 2 * width, n)
            j, k = R1, i
            while i < R1 and j < R2:
                if key(C[A[i]]) > key(C[A[j]]):
                    B[k] = A[j]
                    j += 1
                else:
                    B[k] = A[i]
                    i += 1
                k += 1
            while i < R1:
                B[k] = A[i]
                k += 1
                i += 1
            while k < R2:
                B[k] = A[k]
                k += 1
        A, B = B, A
        width *= 2

    if reverse:
        A.reverse()
    return A


# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", " "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", "\n"))
    if kwargs.pop("flush", False):
        file.flush()


if sys.version_info[0] < 3:
    sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
else:
    sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)

input = lambda: sys.stdin.readline().rstrip("\r\n")

def read():
    return input()

def read_int():
    return int(input())

def read_array(sep=None, maxsplit=-1):
    return input().split(sep, maxsplit)

def read_int_array():
    return [int(a) for a in read_array()]

# endregion

if __name__ == "__main__":
    main()


Measured memory_bytes: 32748