#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-8;
const int INF = 1e9 + 2;
const int mod = 1e9;
const double pi = 3.141592653589793;
int main() {
  int n;
  cin >> n;
  set<pair<int, int> > s;
  for (int i = 0; i < n; i++) {
    int t;
    cin >> t;
    s.insert(make_pair(t, i));
  }
  int v = 0;
  vector<int> ans;
  while (!s.empty()) {
    bool check = false;
    while (v >= 0) {
      auto it = s.upper_bound(make_pair(v, -1));
      it = s.upper_bound(make_pair(v, -1));
      if (it != s.end() && it->first == v) {
        ans.push_back(it->second);
        v++;
        s.erase(it);
        check = true;
      }
      if (check) break;
      v -= 3;
    }
    if (!check) {
      cout << "Impossible";
      return 0;
    }
  }
  cout << "Possible\n";
  for (int i = 0; i < ans.size(); i++) cout << ans[i] + 1 << ' ';
  return 0;
}
