#include <bits/stdc++.h>
using namespace std;
long long binpow(long long a, long long n) {
  long long ans = 1;
  while (n) {
    if (n & 1) ans *= a;
    a *= a;
    n >>= 1;
  }
  return ans;
}
vector<long long> pr;
void aaa(long long x) {
  if (pr.size() > 0) return;
  vector<long long> p(x);
  for (long long i = 2; i <= x; i++) {
    if (p[i] == 0) {
      pr.push_back(i);
      for (long long j = i + i; j <= x; j += i) p[j] = 1;
    }
  }
}
set<long long> st;
long long mx = -1;
long long n;
vector<long long> v1;
vector<long long> v[1000000];
int main() {
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  long long a, pos = 0;
  for (long long i = 0; i < n; ++i) {
    cin >> a;
    v[a].push_back(i + 1);
  }
  while (true) {
    if (n == 0) break;
    if (v[pos].size() > 0) {
      v1.push_back(v[pos].back());
      v[pos].pop_back();
      ++pos;
      --n;
    } else {
      if (pos >= 3)
        pos -= 3;
      else
        break;
    }
    if (n == 0) break;
  }
  if (n == 0) {
    cout << "Possible" << '\n';
    for (long long i = 0; i < v1.size(); ++i) cout << v1[i] << " ";
    cout << '\n';
  } else
    cout << "Impossible" << '\n';
  return 0;
}
