#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for(int i=0;i<(int)(n);i++)

ll dfs(ll x){
	if(x%2==0) return x;
	if(x==1) return 1;
	return dfs((x-1)/2)*2;
}

int main(){
	ll n,m;
	cin>>n>>m;
	vector<ll> a(m);
	rep(i,m) cin>>a.at(i);
	ll ans=0;
	rep(i,m-1){
		ll df=a.at(i+1)-a.at(i)-1;
		if(df==0) continue;
		ans+=dfs(df);
	}
	cout<<ans<<endl;
}