#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int,int> P;
#define REP(i,n) for(int i=0;i<int(n);i++)

ll a[200010];


int main(void){
	cin.tie(nullptr);  ios_base::sync_with_stdio(false);
	ll i,j,k;

	ll N,M;
	cin >> N >> M;
	for(i=1;i<=M;i++) cin >> a[i];

	ll ans=0;
	for(i=2;i<=M;i++){
		ll x=a[i]-a[i-1]-1;

		if(x%2==0){
			ans+=x;
			continue;
		}

		ll e=1;
		while(1){
			if(x==1){
				ans+=e;
				break;
			}

			x--;
			x/=2;
			e*=2;
			if(x%2==0){
				ans+=e*x;
				break;
			}
		}

	}

	cout << ans << endl;

	return 0;
}