//#pragma GCC target("avx,avx2")
//#pragma GCC optimize("Ofast")

#include<bits/stdc++.h>

#define int long long
#define mem(x,y)memset(x,y,sizeof(x))
#define rep(i,l,r)for(int i = l;i <= r;i ++)
#define dep(i,r,l)for(int i = r;i >= l;i --)
#define tov(i)for(int i = head[x];i;i = e[i].nxt)
#define pb push_back
#define pi pair <int,int> 

using namespace std;

int read(){
    int s = 0,w = 1;
    char ch = getchar();
    while(ch < '0' || ch > '9'){if(ch == '-')w = -1;ch = getchar();}
    while(ch >= '0' && ch <= '9')s = s * 10 + ch - '0',ch = getchar();
    return s * w;
}

namespace Tem{
    const int mod = 0;//Notice Here!!!!!!!!!!
    int fac[1000010],ifac[1000010];

    inline int gcd(int a, int b){
        int az = __builtin_ctz(a);
        int bz = __builtin_ctz(b);
        int z = az > bz ? bz : az;
        int diff;
        b >>= bz;
        while(a){
            a >>= az;
            diff = b - a;
            az = __builtin_ctz(diff);
            if(a < b)b = a;
            a = diff < 0 ? -diff : diff;
        }
        return b << z;
    }

    inline int poww(int x,int y,int MOD = mod){
        int s = 1;
        for(;y;x = x * x % MOD,y >>= 1)if(y & 1)s = s * x % MOD;
        return s;
    }

    void initfac(int n,int MOD = mod){
        fac[0] = fac[1] = ifac[0] = ifac[1] = 1;
        rep(i,2,n)fac[i] = fac[i - 1] * i % MOD;
        ifac[n] = poww(fac[n],MOD - 2,MOD);
        dep(i,n - 1,2)ifac[i] = ifac[i + 1] * (i + 1)% MOD;
    }

    inline int inv(int x,int MOD = mod){return poww(x,MOD - 2)% MOD;}

    inline int C(int x,int y,int MOD = mod){return fac[x] * ifac[y] % MOD * ifac[x - y] % MOD;}

    void Add(int &x,int y){x = x + y >= mod ? x + y - mod : x + y;}
    void Dec(int &x,int y){x = x - y < 0 ? x - y + mod : x - y;}
    void Mul(int &x,int y){x = x * y % mod;}
}using namespace Tem;

int n;
int a[1000010];
int cnt[300010];
bool bk[300010];
int tot;
int to[300010];
int b[1000010];
int cnt2[300010];

queue <int> q;

signed main(){
    cin>>n;
    rep(i,1,n)a[i] = read();
    if(n == 1){
        puts("-1");
        return 0;
    }
    rep(i,1,n){
        cnt[a[i]] ++;
        if(cnt[a[i]] == 2)tot ++;
    }
    if(!tot){
        cout<<n + 2<<endl;
        cout<<a[n]<<" ";
        rep(i,1,n - 1)printf("%lld ",a[i]);
        cout<<a[1]<<" "<<a[1];
        return 0;
    }
    int kv = 0;
    rep(i,1,n){
        if(cnt[i] == 1)b[++ kv] = i;
        if(cnt[i] == 0)b[++ kv] = i,b[++ kv] = i;
    }
    rep(i,1,n)if(cnt[a[i]] == 2)bk[i] = 1;
    rep(i,1,n)cnt2[b[i]] ++;
    rep(i,1,n)if(cnt2[i] == 2)q.push(i);
    cout<<n<<endl;
    mem(to,-1);
    int lst;
    lst = q.front();
    q.pop();
    cout<<lst<<" ";
    rep(i,1,n){
        int v = a[i];
        int nxt = a[i + 1];
        if(cnt[v] == 2 && to[v] == -1){
            to[v] = 1;
            if(cnt[nxt] == 2 && to[nxt] == -1){
                int now = q.front();
                q.pop();
                printf("%lld ",now);
            }
            else {
                printf("%lld ",lst);
                i ++;
                q.push(lst);
                int now = q.front();
                q.pop();
                if(i < n - 1)printf("%lld ",now);
                lst = now;
            }
        }
        else printf("%lld ",a[i]);
    }
}