#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;

typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;

typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;

#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)

#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define ins insert

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

const int MOD = 1000000007;
const char nl = '\n';
const int MX = 100001; //check the limits, dummy

int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);

    int N; cin >> N;
    vi A(N); F0R(i, N) cin >> A[i];
    F0R(i, N) {
        if (A[i] > i+1 || (i > 0 && A[i] < A[i-1])) {
            cout << -1 << nl; return 0;
        }
    }
    vi B(N);
    queue<int> need;
    F0Rd(i, N) {
        if ((i > 0 && A[i] > A[i-1]) || (i == 0 && A[i] != 0)) {
            if (i != 0) {

                B[i] = A[i-1];
            } else B[i] = 0;
            FOR(j, A[i-1]+1, A[i]) {
                need.push(j);
            }
        } else {
            if (sz(need) > 0) {
                B[i] = need.front(); need.pop();
            } else {
                B[i] = 1000000;
            }
        }
    }

    F0R(i, N) {
        cout << B[i] << " ";
    }
    cout << nl;

	return 0;
}

// read the question correctly (ll vs int)
// template by bqi343