#include <iostream>
#include <vector>

using namespace std;

int main() {
    int n;
    cin >> n;

    vector<int> tacos(n);
    vector<int> mexValues(n);

    int mex = 0;

    for (int i = 0; i < n; ++i) {
        cin >> tacos[i];

        if (tacos[i] == mex) {
            mexValues[i] = mex;
            mex++;
        } else {
            mexValues[i] = mexValues[i - 1];
        }
    }

    for (int i = 0; i < n; ++i) {
        cout << mexValues[i] << " ";
    }

    return 0;
}