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
import sys
import math
import bisect

def solve(n):
    ans = n * (n + 1) // 2
    if n % 2 == 1:
        tmp = n
    else:
        tmp = n - 1
    #print('n: %d' % (n))
    #print('tmp: %d' % (tmp))
    #print('ans: %d' % (ans))
    ans -= (tmp + 1) * ((tmp - 1) // 2 + 1) // 2
    #print('ans: %d' % (ans))
    ans -= (tmp + 1) * ((tmp - 1) // 2 + 1) // 2
    #print('ans: %d' % (ans))
    return ans

def main():
    n = int(input())
    ans = solve(n)
    print(ans)

if __name__ == "__main__":
    main()
