RT

基本思路:每次看当前能不能读到第 $x$ 号书，有就读，不行把剩下的两个编号最大的卖掉

#pragma GCC optimize(3)

#include<iostream>
#include<algorithm>
#include<cmath>
#include<memory.h>
#include<vector>
#include<queue> 
#include<stack>
#include<cstdlib>

#define int long long 
#define mod 998244353

using namespace std;

int n,in,x=0;
int arr[300005];

deque<int> q;

signed main(){
	
	cin>>n;
	
	for(int i=1;i<=n;i++){
		cin>>arr[i];
	}
	sort(arr+1,arr+n+1,less<int>());
	
	for(int i=1;i<=n;i++){
		q.push_back(arr[i]);
	}
	
	while(1){
		
		if(q.empty()){
			cout<<x<<endl;
			return 0;
		}
		
		if(q.front()!=x+1){
			if(q.size()<2){
				cout<<x<<endl;
				return 0;
			}else{
				q.pop_back();
				q.pop_back();
			}
		}else{
			q.pop_front();
		}
		x++;
	}
	
	return 0;
}