#include <iostream>
#include <sstream>
#include <cstring>
#include <string>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <vector>
#include <queue> 
#include <unordered_set>
#include <unordered_map>
#include <bitset>
#include <ctime>
#include <assert.h>
#include <deque>
#include <list>
#include <stack>


using namespace std;

#define is_mul_overflow(a, b) \
    ((b != 0) && (a > LLONG_MAX / b || a < LLONG_MIN / b))
 
typedef pair<long long, int> pli;
typedef pair<int, long long> pil;
typedef pair<long long , long long> pll;
typedef pair<int, int> pii;
typedef pair<double, double> pdd;
typedef pair<int, pii> piii;
typedef pair<int, long long > pil;
typedef pair<long long, pii> plii;
typedef pair<double, int> pdi;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ull, ull> puu;
typedef long double ld;
const int N = 2000086, MOD = 998244353, INF = 0x3f3f3f3f, MID = 333;
const double EPS = 1e-5;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
// int dx[8] = {1, 1, 0, -1, -1, -1, 0, 1}, dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
// int dx[8] = {2, 1, -1, -2, -2, -1, 1, 2}, dy[8] = {1, 2, 2, 1, -1, -2, -2, -1};
int n, m, cnt;
int w[N];
vector<ll> num;
ll res;

ll lowbit(ll x) { return x & -x; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
inline double rand(double l, double r) { return (double)rand() / RAND_MAX * (r - l) + l; }
inline ll qmi(ll a, ll b, ll c) { ll res = 1; while (b) { if (b & 1) res = res * a % c; a = a * a % c; b >>= 1; } return res; }
inline ll qmi(ll a, ll b) { ll res = 1; while (b) { if (b & 1) res *= a; a *= a; b >>= 1; } return res; }
inline double qmi(double a, ll b) { double res = 1; while (b) { if (b & 1) res *= a; a *= a; b >>= 1; } return res; } 
// inline ll C(ll a, ll b) { if (a < b) return 0; if (b > a - b) b = a - b; ll res = 1; for (ll i = 1, j = a; i <= b; i++, j--) { res = res * (j % MOD) % MOD; res = res * qmi(i, MOD - 2, MOD) % MOD; } return res; }
inline ll C(ll a, ll b, int* c) { if (a < b) return 0; ll res = 1; for (ll j = a, i = 1; i < b + 1; i++, j--) res *= j; for (ll j = a, i = 1; i < b + 1; i++, j--) res /= i; return res; }
inline int find(int x) { return lower_bound(num.begin(), num.end(), x) - num.begin(); }

int c[N];
multiset<int> c1, c2;
set<int> pre;

inline void solve() {
    if (c2.size()) {
        int t = *c2.begin();
        c2.erase(c2.begin());
        printf("%d\n%d ", n, t);
        c[w[1]]--;

        bool flag = 1;
        for (int i = 2; i < n + 1; i++) {
            if (i == 2 && c1.count(w[1])) {
                printf("%d ", w[1]);
                c2.insert(w[1]);
                c[w[2]]--;
                continue;
            }

            if (!flag) {
                flag = 1;
                printf("%d ", t);
                c[w[i]]--;
            } else if (c[w[i]] == 1) {
                printf("%d ", t);
                flag = 0;
            } else if (c[w[i]] == 2) {
                assert(c2.size());
                printf("%d ", *c2.begin());
                c2.erase(c2.begin());
                c[w[i]]--;
            } else assert(0);
        }
    } else {
        printf("%d\n", n + 2);
        printf("%d ", w[n]);
        for (int i = 1; i < n; i++) printf("%d ", w[i]);
        printf("%d %d\n", w[1], w[1]);
    }
}

int main() {
    cin >> n;
    if (n == 1) { puts("-1"); return 0; }

    for (int i = 1; i < n + 1; i++) scanf("%d", w + i), c[w[i]]++;
    for (int i = 1; i < n + 1; i++) {
        if (!c[i]) c2.insert(i);
        else if (c[i] == 1) c1.insert(i);
    }

    solve();
    return 0;
}