#coding:utf-8

from fractions import Fraction

N=input()

S='a'+'ba'*((N-1)/2)

a=S.count('a')-1
b=S.count('b')-1

cnt_a=Fraction(1,2)*a*(a+1)
cnt_b=Fraction(1,2)*b*(b+1)

print cnt_a+cnt_b