#include<bits/stdc++.h>
#define For(i,a,b) for(int i=a;i<=b;++i)
using namespace std;
const int N = 6e5 + 15;
map<int,int> q;
int n,now[N],b[N],a[N],cur,ans[N],bf[N],c[N];
void doit(int x){
	int siz=0,bt=0;
		For(i,x,n+x-1){
			if(now[b[i]]==1||siz==0){
				ans[i]=cur;
				if(siz==0){
					bf[++bt]=cur; now[cur]=1; q[cur]--; siz=1;
				}else{
					while(bf[bt]!=cur) 
						now[bf[bt]]=0,q[bf[bt]]++,--bt;
					now[cur]=0,q[cur]++,--bt;
					siz=0;
				}
			}else{
				int t1=q.begin()->first,t2=q.rbegin()->first;
				if(t1==b[i]){
					ans[i]=t2;
					if(now[t2]){
						while(bf[bt]!=t2) 
							now[bf[bt]]=0,q[bf[bt]]++,--bt;
						now[t2]=0,q[t2]++,--bt;
						continue;
					}
					bf[++bt]=t2; now[t2]=1; q[t2]--; 
					if(!q[t2]) q.erase(t2);
				}else{
					ans[i]=t1;
					if(now[t1]){
						while(bf[bt]!=t1) 
							now[bf[bt]]=0,q[bf[bt]]++,--bt;
						now[t1]=0,q[t1]++,--bt;
					}
					else{
						bf[++bt]=t1; now[t1]=1; q[t1]--; ans[i]=t1;
						if(!q[t1]) q.erase(t1);
					}
				}
			}
			bf[++bt]=b[i];
			now[b[i]]=1;
		//	For(j,1,bt) cout<<bf[j]<<" ";cout<<bt<<bt<<" "<<i<<endl;
		}
		For(i,x,n+x-1) printf("%d ",ans[i]);
}
int main(){
//	freopen("1.in","r",stdin);
//	freopen("1.out","w",stdout);
	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,q[i]=2;
		if(c[i]==1) q[i]=1;
	}
	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]; q[b[1]]++; q.erase(b[2]);
		doit(3);
	}
	return 0;
}