Find the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive). The order does not matter.

Constraints

* 2\leq K\leq 100
* K is an integer.

Input

Input is given from Standard Input in the following format:


K


Output

Print the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive).

Examples

Input

3


Output

2


Input

6


Output

9


Input

11


Output

30


Input

50


Output

625
# vim: set fileencoding=utf-8:


def main():
    K = input()
    even = K / 2
    if K % 2 == 0:
        odd = even
    else:
        odd = even + 1

    print(odd * even)


if __name__ == "__main__":
    main()
