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
import math

num = int(input())

# lastOdd = 0
# lastEven = 0

# negSum = -1
# posSum = 2

# if num%2==0:
#     lastEven = num
#     lastOdd = -num + 1
# else:
#     lastEven = num -1
#     lastOdd = -num

# if num>2:
#     negSum = math.ceil(num / 2) * (lastOdd - 1) / 2;
#     print(negSum)
#     posSum = math.floor(num / 4) * (lastEven + 2);
#     print(posSum)
#     print(posSum + negSum);

if num%2 ==0:
    print(int(num/2))
else:
    print(int(-(num/2+1)))
    