A massive data center has servers numbered from to , each consuming power proportional to its number. To optimize cooling and energy distribution, you need to develop an algorithm to find the server where the total energy consumption from servers to equals the energy consumption from to .

In other words, you must find the number such that the sum from to (inclusive) equals the sum from to (inclusive).

If no such server exists, return . It is guaranteed that there will be at most one of such server for the given input.

Input Format

  • , the number of servers

Constraints

Output Format

The server .

Sample Input 0

8

Sample Output 0

6

Explanation 0

6 is the answer since:

  • 1 + 2 + 3 + 4 + 5 + 6 = 21
  • 6 + 7 + 8 = 21

Sample Input 1

11

Sample Output 1

-1

Explanation 1

We can't find a number that balances the energy.

Line: 1 Col: 1
  1. Challenge Walkthrough
    Let's walk through this sample challenge and explore the features of the code editor.1 of 6
  2. Review the problem statement
    Each challenge has a problem statement that includes sample inputs and outputs. Some challenges include additional information to help you out.2 of 6
  3. Choose a language
    Select the language you wish to use to solve this challenge.3 of 6
  4. Enter your code
    Code your solution in our custom editor or code in your own environment and upload your solution as a file.4 of 6
  5. Test your code
    You can compile your code and test it for errors and accuracy before submitting.5 of 6
  6. Submit to see results
    When you're ready, submit your solution! Remember, you can go back and refine your code anytime.6 of 6
  1. Check your score