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A. Calculating Function

time limit per test

1 secondmemory limit per test

256 megabytesinput

standard inputoutput

standard outputFor a positive integer *n* let's define a function *f*:

*f*(*n*) = - 1 + 2 - 3 + .. + ( - 1)^{ n} *n*

Your task is to calculate *f*(*n*) for a given integer *n*.

Input

The single line contains the positive integer *n* (1 ≤ *n* ≤ 10^{15}).

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

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