import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Main {
    static Map<Long, Long> dp = new HashMap<>();
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        long n = sc.nextLong();
        int m = sc.nextInt();
        dp.put(1L, 1L);
        long prev = sc.nextLong();
        long ans = 0;
        for (int i = 1; i < m; i++) {
            long x = sc.nextLong();
            ans += dfw(x - prev - 1);
            prev = x;
        }
        System.out.println(ans);
    }
    
    static long dfw(long x) {
        if (x == 1) {
            return 1;
        } else if (x % 2 == 0) {
            return x;
        } else {
            return dfw(x / 2) * 2;
        }
    }
}
class Scanner {
    BufferedReader br;
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            
        }
    }
    
    public int nextInt() {
        return Integer.parseInt(next());
    }
    
    public long nextLong() {
        return Long.parseLong(next());
    }
    
    public double nextDouble() {
        return Double.parseDouble(next());
    }
    
    public String next() {
        try {
            while (!st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return st.nextToken();
        }
    }
    
}