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

# Check The inputs for Correctness [Based Of each input's Identifier]
def mCheckInputs(inputList):
    input1 = inputList[0]
    if ((input1.isdigit()) and (1 <= int(input1) <= pow(10,15))):
    	pass
    else:
        exit(1)

# Handle inputs and their Checking and re-Formatting[Str ==> Int]
def mHandleInputs(inputList):
    x = input()
    inputList.append(x)
    mCheckInputs(inputList)

#The Actual Logic [Brain Of Project]
def mDoWork(inputList,outputList): 
        n = int(inputList.pop())
        result = 0

        result = (n//2) if (n % 2 == 0) else ( -(n+1) // 2)
            
        outputList.append(result)

# Re-Format the Result [ How and Where to show it ]
def mFormatResult(outputList):
    print(outputList[0])

def main():
    inputList = [] 
    mHandleInputs(inputList)

    outputList = []
    mDoWork(inputList,outputList)

    mFormatResult(outputList)

if __name__ == "__main__":
    main()
