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 seperateints(x):  
    k=''
    l=[]
    for i in x :
        if i==' ' :
            l.append(int(k))
            k=''
            continue 
        k=k+i
    l.append(int(k))   
    return(l)
def luckynum(x):
    for i in x : 
        if i!='4' and i!='7' :
            return False
    return True 
n=int(input())
if n%2==0 :
    print(n//2)
else :
    print(-((n//2)+1))    