from itertools import count


n = int(input())
count = [0]*(n+1)
ans = []
for i in range(1,n):
    ans.append((i,n))
    count[i] += 1
    count[n] += 1
    for j in range(1,i):
        if count[j] < j:
            ans.append((j,i))
            count[j] += 1
            count[i] += 1
print(len(ans))
for a,b in ans:
    print(a,b)