# Recaman's Sequence

Time Limit: Java: 2000 ms / Others: 2000 ms

Memory Limit: Java: 65536 KB / Others: 65536 KB

## Description

The Recaman's sequence is defined by a0 = 0 ; for m > 0, am = am-1 - m if the rsulting am is positive and not already in the sequence, otherwise am = am-1 + m.

The first few numbers in the Recaman's Sequence is 0, 1, 3, 6, 2, 7, 13, 20, 12, 21, 11, 22, 10, 23, 9... . Given k, your task is to calculate ak.

## Input

The input consists of several test cases. Each line of the input contains an integer k where 0 <= k <= 500000. The last line contains an integer -1, which should not be processed.

## Output

For each k given in the input, print one line containing ak to the output.

## Sample Input

7
10000
-1

## Sample Output

20
18658

None

## Source

Asia 2004, Shanghai (Mainland China), Prelimi