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())
 
even = N//2
odd = N - even

def arithSumEven(n):
    arithSumEven = n*(2*2+(n-1)*2)//2
    return arithSumEven

def arithSumOdd(n):
    arithSumOdd = n*(2*1+(n-1)*2)//2
    return arithSumOdd

print(arithSumEven(even)-arithSumOdd(odd))

 	    		  	 		 	  	   	  		   	