package seq;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Boolean exit = false;
        Scanner sc = new Scanner(System.in);

        Boolean encountered;
        Integer n, x0, ni;
        List<Integer> seq, dif, g1, g2;

        while (!exit) {
            n = Integer.parseInt(sc.nextLine());
            if (n == 0) {
                exit = true;
            } else {
                //Parse input array
                seq = new ArrayList<>();
                for (String x: sc.nextLine().split(" ")) {
                    seq.add(Integer.parseInt(x));
                }

                //Calculate differential array
                x0 = 0;
                dif = new ArrayList<>();
                for (Integer i = 1; i < seq.size(); i++) {
                    Integer x = seq.get(i);
                    dif.add(x - x0);
                    x0 = x;
                }

                //Start seeking for the sequence
                ni = dif.size() / 2;
                g1 = new ArrayList<>();
                g2 = new ArrayList<>();
                encountered = false;
                for (; ni > 1; ni--) {
                    g1 = dif.subList(0, ni);
                    g2 = dif.subList(ni, 2 * ni);

                    if (g1.equals(g2)) {
                        encountered = true;
                        break;
                    }
                }

                if (encountered) {
                    Integer lastNumber = seq.get(seq.size() - 1);
                    Integer addition = dif.get(dif.size() % ni);
                    System.out.println(lastNumber + addition);
                }
            }
        }

        sc.close();
    }

}