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

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N;
    cin >> N;

    vector<int> S(N);
    for (int i = 0; i < N; i++) cin >> S[i];

    queue<int> st;
    int need = 1;

    for (int x : S) {
        while (!st.empty() && st.front() == need) {
            st.pop();
            need++;
        }

        if (x == need) {
            need++;
        } else {
            st.push(x);
        }
    }


    while (!st.empty() && st.front() == need) {
        st.pop();
        need++;
    }

    cout << (need - 1) << "\n";
    return 0;
}