#include <cstdio>
#define re int 
#define maxn 300005

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

int top;
int zhan[maxn];

int main()
{
    scanf("%d", &n);
    re i, j;
    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++){
        for(j = 1;j <= vs[i];j++) zhan[++top] = j;
        if(vs[i] == 2) f = i;
    }
    if(!f) {
        printf("-1\n");
        return 0;
    }
    printf("%d\n", n);
    printf("%d ", f);
    for(i = 1;i < n;i++){
        ok[a[i]] = 1;
        if(ok[a[i + 1]]) {
            printf("%d ", f);
            f = a[i + 1];
        }
        else {
            while(zhan[top] == f && top) top--;
            printf("%d ", zhan[top--]);
        }
    }
    printf("\n");
    return 0;
}
/*
3
1 3 3

1
1

*/