#include <bits/stdc++.h>
using namespace std;
struct number {
  int x, p;
  bool operator<(number y) const { return x < y.x; }
} a[200001];
int s[200001];
int fx[200001];
int as[200001];
int n;
int main() {
  scanf("%d", &n);
  int i;
  for (i = 1; i <= n; i++) {
    scanf("%d", &a[i].x);
    a[i].p = i;
  }
  sort(a + 1, a + 1 + n);
  for (i = 1; i <= n; i++) {
    s[a[i].x]++;
    if (a[i].x != a[i - 1].x || i == 1) fx[a[i].x] = i;
  }
  int d = 0, xt = 0, sum = 0, sx = 0;
  int p = 0;
  while (sx <= n) {
    if (sum < 0 || s[d] == 0) {
      printf("Impossible\n");
      return 0;
    }
    p++;
    as[p] = a[fx[d]].p;
    fx[d]++;
    sx++;
    sum++;
    s[d]--;
    d++;
    if (sx == n) break;
    while (s[d] == 0) {
      sum -= 3;
      d = sum;
      if (d < 0) break;
    }
  }
  printf("Possible\n");
  for (i = 1; i <= p - 1; i++) printf("%d ", as[i]);
  printf("%d\n", as[i]);
  return 0;
}
