#include <iostream>
using namespace std;

int main()
{
    int n, l;
    cin >> n >> l;
    int ans = -1;
    int a[100002];
    cin >> a[0];
    for(int i = 1; i < n; i++){
        cin >> a[i];
        if(a[i - 1] + a[i] >= l) ans = i;
    }
    if(ans == -1) cout << "Impossible" << endl;
    else{
        cout << "Possible" << endl;
        for(int i = 1; i < ans; i++) cout << i << endl;
        for(int i = n - 1; i >= ans; i--) cout << i << endl;
    }
}