import java.util.*;
import java.lang.*;
import java.io.*;

// The main method must be in a class named "Main".
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();
        }
        int m=sc.nextInt();
        int sum=0;
        int count=0;
        for(int i=0;i<n-1;i++){
            if(arr[i]==arr[i+1]){
                sum+=arr[i];
                count++;
            }else{
                if(arr[i]==arr[i-1]){
                    sum+=arr[i];
                    count++;
                    if(count==m){
                        System.out.println(sum);
                        return ;
                    }
                }
                count=0;
                sum=0;
            }
        }
        if(arr[n-1]==arr[n-2]){
            sum+=arr[n-1];
            count++;
             if(count==m){
                        System.out.println(sum);
                        return ;
                    }
        }
        System.out.println(-1);
    }
}