int main()
{
	while(cin >> n && n!=0)
	{
		for(int i=0;i!=88888889;i++)
		check[i] = false;
		float ok = 0.12345678;
		finish = (pow(10.0,n))*ok;
		Pos pos;
		vector<int> init;
		for(int i=0;i!=n;i++)
			init.push_back(0);
		for(int i=0;i!=n;i++)
			pos.push_back(init);
		for(int i=0;i!=n;i++)
		{
			int in;
			cin >> in;
			(pos[i])[0] = in;
		}
		int ans = search(pos,n);
		if(ans==-1)
		cout << "IMPOSSIBLE" <<endl;
		else
		cout << ans <<endl;
	}
	system("pause");
	return 0;
}