#include<bits/stdc++.h>
#define il inline 
#define re register
#define ll long long
#define pb push_back
using namespace std;
const int N=1e6+10;
#define gc() getchar()
il int rd(){
	int x=0,f=1;
	char ch=gc();
	while(!isdigit(ch)){if(ch=='-')f=-1;ch=gc();}
	while(isdigit(ch))x=(x<<3)+(x<<1)+(ch^48),ch=gc();
	return x*f;
}
int n;
int a[N],cnt[N],vis[N];
vector<int>q;
int insert(int x){
	int p=rand()%n+1;
	while(p==q.front()||!cnt[p]||!p==x)p=rand()%n+1;
	--cnt[p],vis[p]=1,q.pb(p);
	return p;
}
void delet(){
	++cnt[q.back()],vis[q.back()]=0,q.pop_back();
}
void Main(){
	n=rd();
	for(int i=1; i<=n; ++i)a[i]=rd();
	srand(time(0));
	if(n==1){
		puts("-1");
		return;
	}
	for(int i=1; i<=n; ++i)++cnt[a[i]];
	for(int i=1; i<=n; ++i)cnt[i]=2-cnt[i];
	for(int i=1; i<=n; ++i){
		if(cnt[i]==2){
			printf("%d\n",n);
			printf("%d ",i);vis[i]=1;q.pb(i),--cnt[i];
			q.pb(a[1]),vis[a[1]]=1;
			for(int j=2; j<=n; ++j){
				if(vis[a[j]]){
					printf("%d ",q.front());
					while(!q.empty())delet();
				}else{
					printf("%d ",insert(a[j]));
				}
				q.pb(a[j]),vis[a[j]]=1;
			}
			exit(0);
		}
	}
	printf("%d\n",n+2);
	printf("%d ",a[1]);
	for(int i=2; i<=n; ++i)printf("%d ",a[i]);
	printf("%d %d",a[n],a[n]);
}
signed main(){
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
	int T=1;
	while(T--)Main();
	return 0;
}