#include <bits/stdc++.h>

int main()
{
	int N;
	int64_t L;
	scanf("%d%lld", &N, &L);
	std::vector<int64_t> a(N);
	for (auto& e: a) scanf("%lld", &e);
	int center{-1};
	for (int i{}; i + 1 < N; i++)
		if (a[i] + a[i + 1] >= L)
		{
			center = i;
			break;
		}
	if (center == -1)
	{
		puts("Impossible");
		return 0;
	}
	puts("Possible");
	for (int i{}; i < center; i++)
		printf("%d\n", i + 1);
	for (int i{N - 2}; i >= center; i--)
		printf("%d\n", i + 1);

	return 0;
}