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
''''
f(n) =  - 1 + 2 - 3 + .. + ( - 1)nn
'''
num = int(input())

ans = None
if num%2 ==0:
    ans = int(num/2)
else :
    ans = -int((num+1)/2)

print(ans)

# count = 0
# for i in range(1,num+1):
#     if i%2 == 0:
#         count += i
#     else :
#         count -= i
#
# print(count)




