#!/bin/python3

import sys

"""def sumOfGroup(k):
    n = 1
    i = n
    sum = 0
    while i < 2*n:
        if i % 2 == 1:
            sum += i
        i += 1
    print(sum) 
    
if __name__ == "__main__":
    k = int(input().strip())
    answer = sumOfGroup(k)
    print(answer)
"""
def sumOfGroup(k):
    numbers = range(0, k)
    total = 0
    for num in numbers:
        if num % 2 == 1:
            total += num
    return total

if __name__ == "__main__":
    k = int(input().strip())
    answer = sumOfGroup(k)
    print(answer)