/* -*- coding: utf-8 -*-
 *
 * 2486.cc:  No.2486 Don't come next to me - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_M = 200000;

/* typedef */

typedef long long ll;

/* global variables */

ll as[MAX_M];

/* subroutines */

/* main */

int main() {
  ll n;
  int m;
  scanf("%lld%d", &n, &m);
  for (int i = 0; i < m; i++) scanf("%lld", as + i);

  ll sum = 0;
  for (int i = 0; i + 1 < m; i++) {
    ll d = as[i + 1] - as[i] - 1, c = 1;
    while (d > 1 && (d & 1)) d >>= 1, c <<= 1;
    sum += d * c;
  }

  printf("%lld\n", sum);

  return 0;
}