#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll mod=1000000007;
ll inf=10000;
ll dfs(ll now){
    if (now%2==0)
    {
        return now;
    }
    if (now==5)
    {
        return 4;
    }else if (now==1)
    {
        return 1;
    }else if (now==3)
    {
        return 2;
    }
    return 2*dfs((now-1)/2);
}
int main(){
    ll n,m;
    cin >> n >> m;
    vector<ll>a(m);
    for (ll i = 0; i < m; i++)
    {
        cin >> a[i];
    }
    ll ans=0;
    for (ll i = 1; i < m; i++)
    {
        ans+=dfs(a[i]-a[i-1]-1);
    }
    cout << ans << endl;
}