#include <bits/stdc++.h>
#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it,X) for(auto it=(X).begin();it!=(X).end();it++)
#define numa(x,a) for(auto x: a)
#define ite iterator
#define mp make_pair
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define pb push_back
#define pf push_front
#define sec second
#define sz(x) ((int)(x).size())
#define ALL( c ) (c).begin(), (c).end()
#define gcd(a,b) __gcd(a,b)
#define mem(x,n) memset(x,n,sizeof(x))
#define endl "\n"
using namespace std;
template <int POS, class TUPLE> void deploy(std::ostream &os, const TUPLE &tuple){}
template <int POS, class TUPLE, class H, class ...Ts> void deploy(std::ostream &os, const TUPLE &t){ os << (POS == 0 ? "" : ", ") << get<POS>(t); deploy<POS + 1, TUPLE, Ts...>(os, t); }
template <class T,class U> std::ostream& operator<<(std::ostream &os, std::pair<T,U> &p){ os << "(" << p.first <<", " << p.second <<")";return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::vector<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::set<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T, class K> std::ostream& operator<<(std::ostream &os, std::map<T, K> &mp){ int remain = mp.size(); os << "{"; for(auto e: mp) os << "(" << e.first << " -> " << e.second << ")" << (--remain == 0 ? "}" : ", "); return os; }
#define DEBUG1(var0) { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG1(var1); }
#define DEBUG3(var0, var1, var2) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG2(var1,var2); }
#define DEBUG4(var0, var1, var2, var3) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG3(var1,var2,var3); }
using ll = long long;

int N;
int nums[55];
bool used[55];
vector <int> OK[55];
int main()
{
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> N;
  rep(i,N){
    int x;
    cin >> x;
    nums[i] = x;
    rep(j,N){
      if (j != x) {
        OK[j].pb(i);
      }
    }
  }
  vector <int> ans;
  rep(i,N){
    int mint = -1;
    int minsz = INF;
    rep(j,N){
      if (used[j]) {
        continue;
      }
      if (minsz > sz(OK[j])) {
        minsz = sz(OK[j]);
        mint = j;
      }
    }
    if (minsz == 0) {
      cout << -1 << endl;
      return 0;
    }
    int man = OK[mint].back();
    used[mint] = true;
    ans.pb(man);
    rep(j,N){
      if (used[j]){
        continue;
      }
      auto it = find(ALL(OK[j]),man);
      if (it != OK[j].end()) {
        OK[j].erase(it);
      }
    }
  }
  cout << sz(ans) << endl;
  rep(i,sz(ans)){
    cout << ans[i] << endl;
  }
  return 0;
}