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
n = int(input())

if n%2 == 0 :
    print (n//2)

else:
    n -= 1
    print ((n//2)*-1 + - 1)

    
    



    



        
        
        
        


            



        
            

    


    

    



        


        



    
        
        

    







    
    
 

    

        
    
            
    




    


        



    





        
            
        
    
