#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, temp;
  cin >> n;
  unordered_map<int, int> mp;
  unordered_map<int, list<int> > mpl;
  for (auto i = 0; i < n; ++i) {
    cin >> temp;
    ++mp[temp];
    mpl[temp].push_back(i + 1);
  }
  list<int> res;
  int n_inside = 0;
  int i = 0;
  while (i < n) {
    if ((mp.find(n_inside) != mp.end()) && (mp[n_inside] > 0)) {
      mp[n_inside]--;
      res.push_back(mpl[n_inside].front());
      mpl[n_inside].erase(mpl[n_inside].begin());
      n_inside++;
      i++;
    } else {
      if (n_inside >= 3)
        n_inside -= 3;
      else {
        cout << "Impossible\n";
        return 0;
      }
    }
  }
  cout << "Possible\n";
  for (auto r : res) cout << r << ' ';
  cout << endl;
  return 0;
}
