You are given an integer array arr$[]$ of size $n,$ representing $n$ number of people in a party, each person is denoted by an integer. Couples are represented by the same number ie: two people have the same integer value; it means they are a couple. Find out the only single person in the party of couples.

NOTE: It is guaranteed that there exists only one single person at the party.

Example $1$:

Input:

$n=5$

arr $=\{1,2,3,2,1\}$

Output:

$3$

Explaination: Only the number $3$ is single.

Example $2$:

Input:

$n=11$

arr$=\{1,2,3,5,3,2,1,4,5,6,6\}$

Output:

$4$

Explaination: $4$ is the only single.

give code in python language.