#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rrep(i, a, b) for(ll i = a; i < (ll)(b); i++)
#define all(x) (x).begin(), (x).end()
#define Ye cout<<"Yes"<<endl
#define YE cout<<"YES"<<endl
#define No cout<<"No"<<endl
#define NO cout<<"NO"<<endl
#define print(n) cout<<n<<endl
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define vec vector
using namespace std;
using ll=long long;
using ull=unsigned long long;
vector<int>dx={0,1,0,-1};
vector<int>dy={1,0,-1,0};
ll mod1=998244353;
ll mod2=1000000007;
int INF=1e9;
ll LINF=-1e18;
int H,W;
vec<string>A;
vec<string>B;
int main(){
    ll N;
    int M;
    cin>>N>>M;
    ll ans=0;
    vec<ll>A(M);
    rep(i,M){
        cin>>A[i];
        if(i){
            ans+=A[i]-A[i-1]-1;
            if((A[i]-A[i-1])%2==0){
                ll point=A[i]-A[i-1]-1;
                ll i=1;
                while(point%2==1&&point>1){
                    point/=2;
                    ans-=i;
                    i*=2;
                }
            }
        }
    }
    print(ans);
}