#include <bits/stdc++.h>
using namespace std;
int a[1000010];
int main()
{
    int n,m;
    cin>>n>>m;
    int ans=0;
    map<int,bool>mp;
    for(int i=1;i<=m;i++)
    {
    	cin>>a[i];
    	if(mp[a[i]-2]==false&&a[i]-2>0)
    		{ans++;mp[a[i]-2]=true;}
    	if(mp[a[i]-1]==false&&a[i]-1>0)	
    		{ans++;mp[a[i]-1]=true;}
    	if(mp[a[i]]==false)
    		{ans++;mp[a[i]]=true;}
    	if(mp[a[i]+1]==false&&a[i]+1<=n)
    		{ans++;mp[a[i]+1]=true;}
    	if(mp[a[i]+2]==false&&a[i]+2<=n)	
    		{ans++;mp[a[i]+2]=true;}
    }
    cout<<ans<<endl;
    return 0;
}