#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001
map<long long,long long> mp;
long long get(long long n){
	if(n<=2)return 0;
	if(n%2==0)return 0;
	return 1 + get(n/2)*2;
}

int main(){
	
	long long n,m;
	cin>>n>>m;
	vector<long long> a(m);
	rep(i,m)cin>>a[i];
	long long ans = n - m;
	rep(i,m-1){
		long long t = a[i+1]-a[i]-1;
		ans -= get(t);
	}
	cout<<ans<<endl;
	
	return 0;
}