Twice-Confirmed Audit Tag Checksum

A warehouse runs two independent stocktake audits over the same shelf, and every scanned item tag code is written into one combined log. Each code appears in that log either once (only one audit spotted the item) or exactly twice (both audits independently confirmed it) — no code appears more than twice. As a lightweight integrity check, the audit system computes the bitwise XOR of every tag code that appears exactly twice in the log. Given the combined log, output that checksum. If no code appears exactly twice, output 0.

Input format

Line 1: an integer n, the number of entries in the log. Line 2: n space-separated integers, the logged tag codes in the order they were scanned.

Output format

Print a single integer: the bitwise XOR of every tag code that appears exactly twice in the log (0 if none does).

Constraints

• 1 <= n <= 1000
• 1 <= code <= 10^9
• Every code appears at most twice in the log.

6
5 7 5 9 7 2

2

3
1 2 3

0