#include<bits/stdc++.h>
#define For(i,a,b) for(int i=a;i<=b;++i)
using namespace std;
const int N = 6e5 + 15;
set<int> ac;
set<int>::iterator it;
int n,now[N],b[N],a[N],cur,ans[N],bf[N],c[N];
void doit(int x){
		int bt=0;
		For(i,x,n+x-1){
			if(now[b[i]]==1||bt==0){
				ans[i]=cur;
				if(bt==0) bf[++bt]=cur,now[cur]=1;
				else{
					while(bf[bt]!=cur){
						ac.insert(bf[bt]);
						now[bf[bt--]]=0;
					}
					now[cur]=0,--bt;
				}
			}else{
				it=ac.begin();
				while((*it)==cur) it++;
				int t=*it;
				ans[i]=t;
				if(now[t]){
					while(bf[bt]!=t){
						ac.insert(bf[bt--]);
						now[bf[bt--]]=0;
					}
					now[t]=0,--bt;
				}else bf[++bt]=t;
			}
			bf[++bt]=b[i];
			now[b[i]]=1;
			if(bt==1) cur=b[i];
		}
		For(i,x,n+x-1) printf("%d ",ans[i]);
}
int main(){
	scanf("%d",&n);
	bool qq=0;
	For(i,1,n){
		scanf("%d",&b[i]); 
		c[b[i]]++;
		if(c[b[i]]>1) qq=1;
	}
	For(i,1,n){
		if(c[i]==0) cur=i;
		if(c[i]<=1) ac.insert(i);
	}
	if(qq){
		printf("%d\n",n);
		doit(1);
	}else{
		if(n==1){
			printf("-1\n");
			return 0;
		}
		printf("%d\n%d %d ",n+2,b[2],b[1]);
		b[n+1]=b[n+2]=b[2]; cur=b[1]; ac.erase(b[2]);
		doit(3);
	}
	return 0;
}