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 function(n):
    sum1=0
    sum2=0
    if n%2==0:
        temp=int(n/2)
        sum1=temp*(temp+1)
        sum2=temp**2
        # print(sum1)
        # print(sum2)
        # print(temp)
    else:
        temp=n//2+1
        sum1=temp*(temp-1)
        sum2=temp**2    
        
        
    print(sum1-sum2)
        
        
n=int(input(''))
function(n)