#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<queue<int> > a(2 * 100 * 1000);
  for (int i = 0; i < n; ++i) {
    int tmp;
    cin >> tmp;
    a[tmp].push(i + 1);
  }
  int cur_num = 0;
  vector<int> res;
  for (int i = 0; i < n; ++i) {
    while (a[cur_num].empty()) {
      if (cur_num >= 3) {
        cur_num -= 3;
      } else {
        cout << "Impossible\n";
        return 0;
      }
    }
    res.push_back(a[cur_num].front());
    a[cur_num].pop();
    ++cur_num;
  }
  cout << "Possible\n";
  for (auto elem : res) {
    cout << elem << " ";
  }
  cout << "\n";
  return 0;
}
