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
def check(n):
    if n%2==0:
        return int(n/2)
    return -(int(n/2)+1)
    
def main():
    # n=input()
    # cnt=0
    # for i in range(int(n)):
    #     m, n = map(int, input().split())
    #     cnt+=check(m,n)
    # print(cnt)
    n=int(input())
    # print(type(n))
    ans=check(n)
    print(ans)
    # m, n = map(int, input().split())
main()