import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

import static java.lang.System.in;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        String[] inputs = reader.readLine().split(" ");
        int N = Integer.parseInt(inputs[0]);
        int M = Integer.parseInt(inputs[1]);
        inputs = reader.readLine().split(" ");
        int top = 1;
        for (int i = M - 1; i > -1; i--) {
            int a = Integer.parseInt(inputs[i]);
            if (top == 1) {
                top = a;
            } else if (top <= a) {
                top -= 1;
            }
        }
        System.out.println(top);
    }
}