import java.util.*;
class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        if(n<0){
            System.out.println("Invalid input");
            return;
        }
        int arr[]=new int[n];
        for(int i=0;i<n;i++){
            arr[i]=sc.nextInt();
            if(arr[i]<0){
                System.out.println("Invalid input");
                return;
            }
        }
        boolean isduplicate=false;
        Arrays.sort(arr);
        for(int i=1;i<n;i++){
            if(arr[i-1]==arr[i]){
                System.out.println(arr[i]);
                isduplicate=true;
            }
        }
        if(isduplicate==false){
            System.out.println("18","-7");
        }
    }
}