#include <iostream>

using namespace std;

typedef long long ll;

ll N, L, x, y, n;

int main() {
  cin >> N >> L >> x;
  bool ans = false;
  for (int i = 0; i < N-1; i++) {
    cin >> y;
    if (x+y >= L) {
      ans = true;
      n = i+1;
    }
    x = y;
  }

  cout << (ans?"Possible":"Impossible") << endl;
  if (ans) {
    for (int i = 1; i < n; i++) cout << i << endl;
    for (int i = N-1; i >= n; i--) cout << i << endl;
  }
  return 0;
}