# Odd Occurrence
## Problem Statement
You are given an array $a$ consisting of $n$ integers. It is guaranteed that exactly one element in the array appears an odd number of times, while all other elements appear an even number of times. Your task is to find and output this unique element.
## Input Format
- First line: $n$ ($1 \le n \le 10^5$) - the number of elements in the array.
- Second line: $n$ integers $a_1, a_2, \dots, a_n$ ($-10^9 \le a_i \le 10^9$) - the elements of the array.
## Output Format
Output a single integer, which is the element that appears an odd number of times.
## Sample Test Cases
### Sample Input 1
```
7
1 2 3 2 1 4 3
```
### Sample Output 1
```
4
```
### Sample Input 2
```
5
-5 0 -5 0 10
```
### Sample Output 2
```
10
```
## Constraints
- $1 \le n \le 10^5$
- $-10^9 \le a_i \le 10^9$
- It is guaranteed that exactly one element appears an odd number of times.
- Time limit: 1 second
- Memory limit: 256 MB
## Explanation
In Sample 1, the given array is $[1, 2, 3, 2, 1, 4, 3]$. The occurrences are:
- $1$ appears $2$ times (even)
- $2$ appears $2$ times (even)
- $3$ appears $2$ times (even)
- $4$ appears $1$ time (odd)
Therefore, $4$ is the element with an odd number of occurrences.
In Sample 2, the given array is $[-5, 0, -5, 0, 10]$. The occurrences are:
- $-5$ appears $2$ times (even)
- $0$ appears $2$ times (even)
- $10$ appears $1$ time (odd)
Thus, $10$ is the element with an odd number of occurrences.
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