1162C - Hide and Seek - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
#define mem(a,b) memset(a,b,sizeof a)
#define PII pair<int,int>
#define ll long long
#define ull unsigned long long
#define IOS ios::sync_with_stdio(false)
#define gcd(a,b) __gcd(a,b)
#define ft first
#define sd second
#define endl '\n'
#define PI acos(-1.0)
#define lcm(a,b) a/gcd(a,b)*b
#define INF_INT 0x3f3f3f3f
#define INF_LONG 4557430888798830399
#define rep(i,x,y) for(int i=x; i<=y; ++i)
#define pre(i,x,y) for(int i=x; i>=y; --i)
#define pushk push_back
#define popk pop_back
#define mod 1000000007
#define lowbit(a) ((a)&-(a))
const int N = 1e5 + 1000;
ll a[N];
ll q[N],h[N];
int main(){
	IOS;
	int n,m;
	cin>>n>>m;
	memset(q,127,sizeof(q));
	for(ll i = 1;i <= m;i ++){
		cin>>a[i];
		q[a[i]] = min(q[a[i]],i);
		h[a[i]] = max(h[a[i]],i);
	}
	set<pair<int,int>>se;
	for(int i = 1;i <= n;i ++){
		if(i == 1){
			se.insert({1,1});
			if(1 + 1 <= n)se.insert({1,2});
		} else {
			se.insert({i,i});
			se.insert({i,i - 1});
			if(i + 1 <= n)se.insert({i,i + 1});			
		}
	}
	ll da = se.size();
	for(auto to : se){

		if(q[to.first] <= h[to.second]){
//			cout<<to.first<<" "<<to.second<<endl;
			da--;
		}
		
	}
	cout<<da<<endl;
	return 0;
}