#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef pair<int,int> P;
typedef long double ld;

int main(){
	ll n;
	int m;
	cin >> n >> m;
	vl a(m + 2);
	a[0] = 0;
	rep(i, m) cin >> a[i + 1];
	a[m + 1] = n + 1;
	ll ans = 0;
	rep(i, m + 1){
		ll d = a[i + 1] - a[i] - 1;
		if(d <= 0) continue;
		ll c = 1;
		while(d >= 3 and d % 2 == 1){
			d--;
			d /= 2;
			c *= 2;
		}
		ans += c * d;
	}
	cout << ans;
	return 0;
}