#include <bits/stdc++.h>
using namespace std;
const int N = 6e5 + 5;
int n, a[N], f = 1, c[N], x, t, q[N], ban[N];
int main() {
	cin >> n;
	for (int i=1; i<=n; i++)
		scanf ("%d", &a[i]), f &= ++ c[a[i]] & 1;
	if (f)
		if (n == 1) puts ("-1");
		else {
			printf ("%d\n%d", n + 2, a[n]);
			for (int i=1; i<n; i++) printf (" %d", a[i]);
			printf (" %d %d", a[1], a[1]);
		}
	else { printf ("%d\n", n);
		for (int i=1; i<=n; i++)
			if (! c[i] && ! x) x = i;
			else for (int j=c[i]; j<2; j++)
				q[++t] = j;
		for (int i=1, j=1; i<=n; i++) {
			if (! f) printf ("%d ", x), f = 1;
			else if (c[a[i]] == 2) {
				while (ban[q[j]]) j ++;
				printf ("%d ", q[j++]);
			} else printf ("%d ", x), f = 0, ban[a[i]] = 1;
			c[a[i]] --;
		}
	}
}