#include <iostream>
#include <vector>
using namespace std;
int main(){
    int n;cin>>n;
    vector<int> A(110000);
    for(int i = 0; n > i; i++){
        int x;cin>>x;
        A[x]++;
    }
    bool am = false;
    bool lck = false;
    int i = 0;
    while(true){
        if(A[i] == 1 && (i+1)*2 <= n && !lck){
            lck = true;
        }else if(A[i] < 2){
            cout << i << endl;
            return 0;
        }
        i++;
    }
}