#include<bits/stdc++.h>
using namespace std;
using ll=long long;
using pii=pair<int,int>;

#define all(a) a.begin(),a.end()
#define pb push_back
#define sz(a) ((int)a.size())

const int N=200005;
ll n,a[N];
int m;

signed main(){
    ios_base::sync_with_stdio(0),cin.tie(0);
    cin >> n >> m;
    for(int i=0; i<m; ++i) cin >> a[i];
    ll res=0;
    for(int i=0; i<m-1; ++i){
        ll x=a[i+1]-a[i]-1,cnt=0;
        for(int j=0; j<77; ++j){
            if(~x>>j&1) break;
            cnt+=1ll<<j;
        }
        if(cnt==x) res+=(x+1)/2;
        else res+=x-cnt;
    }
    cout << res << "\n";
}