//#include<bits/stdc++.h>
#include<algorithm>
#include <iostream>
#include <fstream>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <cstdio>
#include <vector>
#include <string>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
using namespace std;
#define P(a,b,c) make_pair(a,make_pair(b,c))
#define rep(i,a,n) for (int i=a;i<=n;i++)
#define per(i,a,n) for (int i=n;i>=a;i--)
#define CLR(vis) memset(vis,0,sizeof(vis))
#define MST(vis,pos) memset(vis,pos,sizeof(vis))
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef pair<int,pair<int,int> >pii;
typedef long long ll;
typedef unsigned long long ull;
const ll mod = 1000000007;
const int INF = 0x3f3f3f3f;
ll gcd(ll a, ll b) {
return b ? gcd(b, a%b) : a;
}
template<class T>inline void gmax(T &A, T B) {
(A<B)&&(A=B);//if(B>A)A=B;
}
template<class T>inline void gmin(T &A, T B) {
(A>B)&&(A=B);//if(B<A)A=B;
}
template <class T>
inline bool scan_d(T &ret) {
char c;
int sgn;
if(c=getchar(),c==EOF) return 0; //EOF
while(c!='-'&&(c<'0'||c>'9')) c=getchar();
sgn=(c=='-')?-1:1;
ret=(c=='-')?0:(c-'0');
while(c=getchar(),c>='0'&&c<='9') ret=ret*10+(c-'0');
ret*=sgn;
return 1;
}
inline void outt(int x) {
if(x>9) outt(x/10);
putchar(x%10+'0');
}
const int MAXN = 1e6+10;
int a[MAXN];
int cnt[MAXN];
int vis1[MAXN];
int vis2[MAXN];
int vis3[MAXN];
int main() {
int n,k;
cin>>n>>k;
rep(i,1,n){
cnt[i]=3;
vis1[i]=0,vis2[i]=0,vis3[i]=0;
}
rep(i,1,k)scan_d(a[i]);
rep(i,1,k){
if(!vis1[a[i]]){
vis1[a[i]]=1;cnt[a[i]]--;
}
if(vis1[a[i]-1]&&!vis2[a[i]-1]){
cnt[a[i]-1]--;
vis2[a[i]-1]=1;
}
if(vis1[a[i]+1]&&!vis3[a[i]+1]){
cnt[a[i]+1]--;
vis3[a[i]+1]=1;
}
}
ll sum=0;
rep(i,1,n){
sum+=cnt[i];
}
cout<<sum-2<<endl;
return 0;
}
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