# import sys
# input = sys.stdin.readline
def mp():return map(int,input().split())
def lmp():return list(map(int,input().split()))
import math
import bisect
from copy import deepcopy as dc
from itertools import accumulate
from collections import Counter, defaultdict, deque
def ceil(U,V):return (U+V-1)//V
def modf1(N,MOD):return (N-1)%MOD+1
inf = int(1e30)
mod = int(1e9+7)

n = int(input())
used = [set() for i in range(n+1)]
m = 0
for i in range(1,n+1):
    cnt = n-i-len(used[i])
    #print(cnt)
    for j in range(1,n+1):
        if i != j and j not in used[i] and cnt>=1:
            m += 1
            cnt -= 1
            used[i].add(j)
            used[j].add(i)
    #print(used)
print(m)
for i in range(1,n+1):
    for j in used[i]:
        if i < j:print(i,j)