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
#calculating_function.py
'''
for k in range(1,20):
    
    n = k
    total = 0
    for i in range(1,n+1):
        if i%2 == 0:
            total += i
        else:
            total -= i
    print(i,total)
'''

n = int(input())
if n%2 == 0:
    print(n//2)

else:
    print(-(n+1)//2)


