#include <cstdio>
#include <map>
using namespace std;
#define re int 
#define maxn 300005

int n;
int a[maxn];
int vs[maxn];
int ok[maxn];

int top;
int zhan[maxn];

map<int, int> mp;

int main()
{
    scanf("%d", &n);
    re i;
    for(i = 1;i <= n;i++) vs[i] = 2;
    for(i = 1;i <= n;i++) scanf("%d", &a[i]), vs[a[i]]--;
    if(n == 1){
        printf("-1\n");
        return 0;
    }
    int f = 0;
    for(i = 1;i <= n;i++){
        if(vs[i]) mp[i] = vs[i];
        if(vs[i] == 2) f = i;
    }
    if(!f) {
        printf("-1\n");
        return 0;
    }
    printf("%d\n", n);
    printf("%d ", f);
    map<int, int>::iterator it;
    zhan[++top] = f, ok[f] = 1, mp[f]--;
    int x;
    for(i = 1;i < n;i++){
        ok[a[i]] = 1, zhan[++top] = a[i];
        if(ok[a[i + 1]]){
            printf("%d ", f);
            while(zhan[top] != f) {
                mp[zhan[top]]++, ok[zhan[top]] = 0, top--;
            }
            mp[zhan[top]]++, ok[zhan[top]] = 0, top--;
            f = a[i + 1];
        }
        else {
            it = mp.end();
            it--;
            while(it -> first == a[i + 1] || (it -> first == f && !mp[a[i + 1]])) it--;
            if(it -> first == f) {
            	while(zhan[top] != f) {
                	mp[zhan[top]]++, ok[zhan[top]] = 0, top--;
            	}
            	mp[zhan[top]]++, ok[zhan[top]] = 0, top--;
				f = a[i + 1];
			}
            x = it -> first;
            printf("%d ", x);
            mp[x]--, zhan[++top] = x, ok[x] = 1;
            if(mp[x] == 0) 
				mp.erase(x);
			
        }
    }
    return 0;
}
/*
3
1 3 3

1
1

5
1 2 3 4 4

8
1 2 2 3 4 5 3 1

5
1 2 4 4 3
*/