import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))


n = int(input())
es = []
ds = [0]*n
for i in range(n-2, 0, -1):
#     print(i, ds[i], es)
    if ds[i]>=i:
        break
    ds[i] += 1
    ds[n-1] += 1
    es.append((i,n-1))
    j = i-1
    assert i!=n-1
    while ds[i]<i:
        es.append((j,i))
        assert i!=j
        ds[i] += 1
        ds[j] += 1
        j -= 1
assert len(set(ds))==n-1
if n==2:
    es = [(0,1)]
print(len(es))
write("\n".join([" ".join(map(str, [v+1 for v in item])) for item in es]))