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 main():
    #leitura
    n = int(input())
    if n % 2 == 0:
        ultimo_par = n
        ultimo_impar = n-1
        soma_impar = ((1 + ultimo_impar)*(n//2))//2
        soma_par = ((2 + ultimo_par)*(n//2))//2
    else:
        ultimo_par = n-1
        ultimo_impar = n
        soma_impar = ((1 + ultimo_impar)*(n//2 + 1))//2
        soma_par = ((2 + ultimo_par)*(n//2))//2

    f = soma_par - soma_impar
    print(f)

main()
  		 	 	  		    	 	 	 		  		