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
'''
A. Calculating Function
time limit per test1 second
memory limit per test256 megabytes
inputstandard input
outputstandard output
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
inputCopy
4
outputCopy
2
inputCopy
5
outputCopy
-3
Note
f(4) =  - 1 + 2 - 3 + 4 = 2

f(5) =  - 1 + 2 - 3 + 4 - 5 =  - 3


'''
CALC_INPUT = int(input())
CALC_TMP = CALC_INPUT / 2.0
if int(CALC_TMP) == CALC_TMP:
    print(int(CALC_TMP))
else:
    print(0-(CALC_INPUT - int(CALC_TMP)))