For a positive integer n let's define a function f:

f(n) = - 1 + 2 - 3 + .. + ( - 1)nn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1 ≤ n ≤ 1015).

Output

Print f(n) in a single line.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

f(4) = - 1 + 2 - 3 + 4 = 2

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
#!/usr/bin/python
import sys

"""
def calculation1(n):
    F = - 1
    P = 1
    res = 0
    while P <= n:
        res += P * F
        P += 1
        F = - F
    return res
"""

def calculation(n):
    if n & 1 == 0: return (n + 1) / 2
    else: return -((n + 1)/ 2)

if __name__ == '__main__':
    n = int(sys.stdin.readline())
    print calculation(n)
    #for i in range(1, 101):
    #    print calculation1(i), calculation2(i)