#include<bits/stdc++.h>
#include<atcoder/all>
using namespace atcoder;
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
using ll=long long;
using ld=long double;
using ull=unsigned long long;
using i128 = __int128_t;
#define vs vector<string>
#define rep(i,n) for(ll i=0;i<ll(n);i++)
#define rep2(i,n) for(ll i=n-1;i>=0;i--)
#define rep3(i,a,b) for(ll i=a;i<=ll(b);i++)
#define rep4(i,a,b) for(ll i=a;i>=ll(b);i--)
#define forv(i,V) for(const auto& i:V)
#define all(x) x.begin(),x.end()
#define fi first
#define se second
#define SIZE(x) int(x.size())
constexpr ll mod=998244353;
//constexpr ll mod=1000000007;
#define pi 3.14159265358979323
#define INF32 2147483647
#define INF64 9223372036854775807
#define faster ios::sync_with_stdio(false);std::cin.tie(nullptr)
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define rev reverse
#define vi vector<int>
#define vll vector<ll>
#define vpi vector<pair<int,int>>
#define vpll vector<pair<ll,ll>>
#define vvi vector<vector<int>>
#define vvll vector<vector<ll>>
#define prq priority_queue
#define lb lower_bound
#define ub upper_bound
#define popcnt __builtin_popcountll
#define TLE while(true);
#define RE assert(false);
#define MLE vector<vector<vector<long long>>> mle_mle(400,vector<vector<long long>>(1000,vector<long long>(1000)));
const string YESNO[2] = {"NO", "YES"};
const string YesNo[2] = {"No", "Yes"};
const string yesno[2] = {"no", "yes"};
#define rall(n) (n).rbegin(),(n).rend()
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;scan(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;scan(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;scan(__VA_ARGS__)
template<typename T,typename U>
ostream &operator<<(ostream&os,const pair<T,U>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T,typename U>
istream &operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<typename T>
ostream &operator<<(ostream&os,const vector<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<typename T>
istream &operator>>(istream&is,vector<T>&v){for(T &in:v){is>>in;}return is;}
void scan(){}
template<class Head,class... Tail>
void scan(Head&head,Tail&... tail){cin>>head;scan(tail...);}
template<class T>
void print(const T &t){cout<<t<<'\n';}
template<class Head, class... Tail>
void print(const Head &head, const Tail &... tail){cout<<head<<' ';print(tail...);}
template<class... T>
void fin(const T &... a){print(a...);exit(0);}
ll max(int a,ll b){return max((ll)a,b);}
ll max(ll a,int b){return max((ll)b,a);}
ll min(int a,ll b){return min((ll)a,b);}
ll min(ll a,int b){return min((ll)b,a);}
//a以上b以下の個数
template<typename T>
ll RangeOK(ll a,ll b,vector<T> &v){
  return max(ub(all(v),b)-lb(all(v),a),0);
}
template <typename T>
vector<T> compress(vector<T> &X) {
    vector<T> vals = X;
    sort(vals.begin(), vals.end());
    vals.erase(unique(vals.begin(), vals.end()), vals.end());
    for (int i = 0; i < (int)X.size(); i++) {
        X[i] = lower_bound(vals.begin(), vals.end(), X[i]) - vals.begin();
    }
    return vals;
}
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
int main(){
  LL(n, m);
  vll v(m);
  scan(v);
  vll d;
  rep(i, m-1) d.pb(v[i+1]-v[i]-1);
  ll ans = 0;
  for(auto x:d){
    ll p = 1;
    while(x%2){
      if(x == 1){
        break;
      }else{
        p*=2;
      }
      x/=2;
    }
    x *= p;
    ans += x;
  }
  print(ans);
}