#include <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int>A(N),B(200200);
    for(int i = 0; i < N; i++) {
        cin >> A[i];
        B[A[i]]++;
    }
    int f = 1,f2 = 0;
    for(int i = 0; i <= 200000; i++) {
        if(B[i] == 0) {
            cout << i << endl;
            return 0;
        }
        if(B[i] == 2) {
            f2++;
            if(f == 0 && f2*2+1 == N) {
                cout << i << endl;
                return 0;
            }
            continue;
        }
        if(B[i] == 1 && f == 1) {
            if(f2*2+1 == N) {
                cout << i << endl;
                return 0;
            }
            f = 0;
        }
        else if(B[i] == 1) {
            cout << i << endl;
            return 0;
        }
    }
}