/* Problem URL: https://codeforces.com/contest/911/problem/E */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template <class T, class comp = less<>>
using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (int i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (int i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

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

    int n, k;
    cin >> n >> k;

    vi fds(k);
    cin >> fds;
    
    vi ans;

    stack<int> s;

    rep(i, k) {
        if (ans.size() == fds[i] - 1) {
            ans.push_back(fds[i]);

            while (!s.empty() && s.top() == ans.size() + 1) {
                ans.push_back(s.top());
                s.pop();
            }

            continue;
        }

        if (!s.empty() && s.top() < fds[i]) {
            cout << "-1\n";
            return 0;
        }

        s.push(fds[i]);
    }

    while (!s.empty() && s.top() == ans.size() - 1) {
        ans.push_back(s.top());
        s.pop();
    }

    int minimal = ans.size() + 1;
    int count = ans.size();

    while (!s.empty()) {
        if (s.top() != count + 1) {
            for (int i = s.top() - 1; i >= minimal; i--) {
                fds.push_back(i);
                count++;
            }
        }

        count++;
        minimal = s.top() + 1;
        s.pop();
    }

    int now = 0;

    while (fds.size() < n) {
        fds.push_back(n - now);
        now++;
    }

    cout << fds;
}