import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int [] arr=new int[n];
        for(int i=0;i<n;i++){
            arr[i]=sc.nextInt();
        }
        Hashset<Integer>set=new Hashset<>();
        for(int i=1;i<n;i++){
            sum=arr[i];
            if(set.contains(arr[i])){
                System.out.print(arr[i]);
                return;
            }
            set.add(arr[i]);
        }
        System.out.print(-1);
    }
}