import sys
from typing import List, Tuple
import random

int1 = lambda x: int(x) - 1
# input = lambda: sys.stdin.readline().rstrip('\n')
ii = lambda: int(input())
vi = lambda: list(map(int, input().split()))
vi1 = lambda: list(map(int1, input().split()))


def dbg(*args, **kwargs):
    print(*(repr(arg) for arg in args), *(f'{k}: {repr(v)}' for k, v in kwargs.items()),
          sep='; ', file=sys.stderr, flush=True)


def main():
    n = int(input())
    if n <= 2:
        return -1
    nn = n * n
    print(nn * 2)
    for i in range(nn):
        for j in [(i - n) % nn, (i - 1) % nn, (i + 1) % nn, (i + n) % nn]:
            if i < j:
                print(i + 1, j + 1)


def _start():
    if (ret := main()) is not None:
        print(*ret) if isinstance(ret, List) or isinstance(ret, Tuple) else print(ret)


if __name__ == '__main__':
    _start()