import java.util.*;

public class Test_1874 {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner s = new Scanner(System.in);
        int n, k, m;

        n = s.nextInt();
        k = s.nextInt();

        long[] a = new long[k];
        for (int i = 0; i < k; i++) {
            a[i] = s.nextInt();
        }
        for (int i = 1; i < k; i++) {
            if (a[i] < a[i - 1]) {
                m = (int) a[i];
                a[i] = a[i - 1];
                a[i - 1] = m;
            }
        }
        m = (int) (n - (n - a[0]) - (n - a[1]));
        if (m <= 0) {
            System.out.println("0");
        } else {
            System.out.println(m);
        }
    }
}