#include<bits/stdc++.h>
using namespace std;
int n,a,b,ans;
bool t[100001];
int main()
{
	cin>>n>>a;
	for(int i=1;i<=a;i++){
	cin>>b;
	for(int j=b-1;j<=b+3;j++)t[j] = 1;
	}
	for(int i=2;i<=n+1;i++)if(t[i])ans++;
	cout<<ans;
	return 0;
}