#311. Alternating sum
You get n. Find the sum 1 - 2 + 3 - 4 + ... ± n.
### Input
One integer n (1 ≤ n ≤ 1000).
### Output
One integer.
Examples
Input
304
Output
-152
Input
262
Output
-131
💡 You can solve it with Python, JavaScript or blocks. Read the input line by line with input(). Don't print a question (like input("Number: ")). Print exactly what the output asks for, nothing more.