#include <bits/stdc++.h>
using namespace std;
long long powmod(long long a, long long b) {
  long long res = 1;
  a %= 1000000007;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % 1000000007;
    a = a * a % 1000000007;
  }
  return res % 1000000007;
}
vector<int> mm[1000005];
vector<int> v;
int main() {
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
    mm[arr[i]].push_back(i + 1);
  }
  int curr = -1;
  for (int i = 0; i < n; i++) {
    if (curr >= -1 && mm[curr + 1].size() > 0) {
      v.push_back(mm[curr + 1].back());
      mm[curr + 1].pop_back();
      curr += 1;
    } else if (curr < 0) {
      cout << "Impossible" << endl;
      return 0;
    } else {
      curr -= 3;
      i--;
    }
  }
  cout << "Possible" << endl;
  for (int i = 0; i < v.size(); i++) {
    cout << v[i] << " ";
  }
  return 0;
}
