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 fd70000000():
    n = int(input())
    if n%2:
        return(print(-(n//2+1)))
    else:    
        return(print(n//2))

fd70000000()

#-1 +1 -2 +2 -3 +3 -4 +4 -5 +5
# 1  2  3  4  5  6  7  8  9  10
# if even out = n//2
# if odd  out = -(n//2+1)
