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

int main(){
	ll n,m;
	cin>>n>>m;
	vector<ll> a(m);
	rep(i,m) cin>>a.at(i);
	ll ans=0;
	if(a.back()==n-1) ans++;
	if(a.back()<=n-1){
		a.push_back(n);
		m++;
	}
	if(a.at(0)==2) ans++;
	if(a.at(0)>=2){
		a.push_back(1);
		m++;
	}
	sort(a.begin(),a.end());
	rep(i,m-1){
		ll df=a.at(i+1)-a.at(i)-1;
		if(df==0) continue;
		if(df!=1){
			if(df&1) df--;
		}
		ans+=df;
	}
	cout<<ans<<endl;
}