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

public class Main {

    static Integer size, maxSize;
    static ArrayList<Integer> points;

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);

        size = in.nextInt();
        maxSize = in.nextInt();

        points = new ArrayList<>(size);

        for (int i = 0; i < size; i++) {
            points.add(in.nextInt());
        }

        Integer answ = 1;

        while (true) {

            Integer t = answ + points.get(0);
            Integer steps = 1;
            Boolean found = false;

            while (true) {

                if (!points.contains(t))
                    break;
                else if (points.indexOf(t) == points.size() - 1){

                    found = true;
                    break;

                }else{

                    t += answ;
                    steps++;

                }

            }

            if(found && steps <= maxSize)
                break;

            answ ++;

        }

        System.out.println(answ);

    }
}