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
# =============================
#   Problem - 160A - Twins
#       Python3
#   Author - wdaubry
# =============================
 
def main():
    from sys import stdout
 
    n = int(input())
 
    if n%2 == 0:
        res = int(n/2)
    else:
        res = int(-(n+1)/2)
 
    stdout.write(str(res))
 
if __name__ == '__main__':
    main()