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
n = int(input())

# Case even n:
    # n = 4: -1 + 2 - 3 + 4 = 2 = n / 2 
    # n = 6: -1 + 2 - 3 + 4 - 5 + 6 = 3 = n / 2
   
# Case odd n:
    # n = 5: -1 + 2 - 3 + 4 - 5 = -3 = n // 2 - n
    # n = 7: -1 + 2 - 3 + 4 - 5 + 6 - 7 = n // 2 - n

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