# -*- coding: utf-8 -*-

import bisect

import heapq

import math

import random

import sys

from collections import Counter, defaultdict, deque

from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal

from functools import lru_cache, reduce

from itertools import combinations, combinations_with_replacement, product, permutations

from operator import add, mul, sub



sys.setrecursionlimit(100000)





def read_int():

    return int(input())





def read_int_n():

    return list(map(int, input().split()))





def read_float():

    return float(input())





def read_float_n():

    return list(map(float, input().split()))





def read_str():

    return input().strip()





def read_str_n():

    return list(map(str, input().split()))





def error_print(*args):

    print(*args, file=sys.stderr)





def mt(f):

    import time



    def wrap(*args, **kwargs):

        s = time.time()

        ret = f(*args, **kwargs)

        e = time.time()



        error_print(e - s, 'sec')

        return ret



    return wrap





@mt

def slv(N):

    E = []

    

    for i in range(1, N+1):

        for j in range(i+1, N+1):

            if N % 2 == 0 and i+j == N+1:

                continue

            if N % 2 == 1 and i+j == N:

                continue

            E.append((i, j))



    return E





def main():

    N = read_int()

    E = slv(N)

    print(len(E))

    for e in E:

        print('%d %d' % e)





if __name__ == '__main__':

    main()

# -*- coding: utf-8 -*-

import bisect

import heapq

import math

import random

import sys

from collections import Counter, defaultdict, deque

from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal

from functools import lru_cache, reduce

from itertools import combinations, combinations_with_replacement, product, permutations

from operator import add, mul, sub



sys.setrecursionlimit(100000)





def read_int():

    return int(input())





def read_int_n():

    return list(map(int, input().split()))





def read_float():

    return float(input())





def read_float_n():

    return list(map(float, input().split()))





def read_str():

    return input().strip()





def read_str_n():

    return list(map(str, input().split()))





def error_print(*args):

    print(*args, file=sys.stderr)





def mt(f):

    import time



    def wrap(*args, **kwargs):

        s = time.time()

        ret = f(*args, **kwargs)

        e = time.time()



        error_print(e - s, 'sec')

        return ret



    return wrap





@mt

def slv(N):

    se = N+1 if N % 2 == 0 else N

    return list(filter(lambda x: x[0]+x[1]!=se, combinations(range(1, N+1), 2)))





def main():

    N = read_int()

    E = slv(N)

    print(len(E))

    for e in E:

        print('%d %d' % e)





if __name__ == '__main__':

    main()

# -*- coding: utf-8 -*-
import bisect
import heapq
import math
import random
import sys
from collections import Counter, defaultdict, deque
from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal
from functools import lru_cache, reduce
from itertools import combinations, combinations_with_replacement, product, permutations
from operator import add, mul, sub

sys.setrecursionlimit(100000)


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


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input().strip()


def read_str_n():
    return list(map(str, input().split()))


def error_print(*args):
    print(*args, file=sys.stderr)


def mt(f):
    import time

    def wrap(*args, **kwargs):
        s = time.time()
        ret = f(*args, **kwargs)
        e = time.time()
        error_print(e - s, "sec")
        return ret

    return wrap


@mt
def slv(N):
    E = []
    for i in range(1, N + 1):
        for j in range(i + 1, N + 1):
            if N % 2 == 0 and i + j == N + 1:
                continue
            if N % 2 == 1 and i + j == N:
                continue
            E.append((i, j))
    return E


def main():
    N = read_int()
    E = slv(N)
    print(len(E))
    for e in E:
        print("%d %d" % e)


if __name__ == "__main__":
    main()

# -*- coding: utf-8 -*-
import bisect
import heapq
import math
import random
import sys
from collections import Counter, defaultdict, deque
from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal
from functools import lru_cache, reduce
from itertools import combinations, combinations_with_replacement, product, permutations
from operator import add, mul, sub

sys.setrecursionlimit(100000)


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


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input().strip()


def read_str_n():
    return list(map(str, input().split()))


def error_print(*args):
    print(*args, file=sys.stderr)


def mt(f):
    import time

    def wrap(*args, **kwargs):
        s = time.time()
        ret = f(*args, **kwargs)
        e = time.time()
        error_print(e - s, "sec")
        return ret

    return wrap


@mt
def slv(N):
    se = N + 1 if N % 2 == 0 else N
    return list(filter(lambda x: x[0] + x[1] != se, combinations(range(1, N + 1), 2)))


def main():
    N = read_int()
    E = slv(N)
    print(len(E))
    for e in E:
        print("%d %d" % e)


if __name__ == "__main__":
    main()

-    E = [] -    for i in range(1, N + 1): -        for j in range(i + 1, N + 1): -            if N % 2 == 0 and i + j == N + 1: -                continue -            if N % 2 == 1 and i + j == N: -                continue -            E.append((i, j)) -    return E +    se = N + 1 if N % 2 == 0 else N +    return list(filter(lambda x: x[0] + x[1] != se, combinations(range(1, N + 1), 2)))