import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc= new Scanner(System.in);
        int n=sc.nextInt();
         if(n<=0){
            System.out.print("Invalid input");
        }
        int arr[] = new int[n];
        for(int i=0; i<arr.length;i++){
            
            if(!sc.hasNextint()){
            System.out.print("Invalid input");
            return;
        }
            arr[i]=sc.nextInt();
        }
        int c =0;
        for(int i=0; i<n;i++){
            for(int j=i+1;j<n;j++){
                if(arr[i]==arr[j]){
                    System.out.print(arr[i]);
                    c++;
                  
                }
            }
        }
        if (c<=0){
            System.out.print("-1");
        }
    }
}