#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
void solve() {
  int n;
  cin >> n;
  multiset<int> mt;
  vector<int> v[n];
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    mt.insert(x);
    v[x].push_back(i + 1);
  }
  int cur = -1, cnt = 0;
  vector<int> ans;
  while (cnt < n) {
    cur++;
    while (cur >= 0 and mt.find(cur) == mt.end()) {
      cur -= 3;
    }
    if (cur < 0) {
      cout << "Impossible\n";
      return;
    }
    mt.erase(mt.lower_bound(cur));
    ans.push_back(v[cur].back());
    v[cur].pop_back();
    cnt++;
  }
  cout << "Possible\n";
  for (auto i : ans) cout << i << " ";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  solve();
}
