C++ :

#include <bits/stdc++.h>
using namespace std;

int a[10000010],n,i,j;
int main(){
	cin>>n;
	for(i = 0;i < n;i++){
		cin>>a[i];
	}
	
	sort(a,a+n);
	int c = 0;
	for(i = 0;i < n;i++){
		c++;
		if(i == n - 1 || a[i] != a[i + 1]){
			if(c % 2 == 1){
				cout<<a[i]<<endl;
				break;
			}
			
			c = 0;
		}
	}
}

Python :

import math
import sys
n=int(input())
m=list(map(int,input().split()))
d=dict()
for item  in m:
    if item not in d:
        d[item]=1
    else:
        d[item]=d[item]+1

for item in d.keys():
    if d[item]%2!=0:
        print(item)
        sys.exit()