CF576D.Flights for Regular Customers

In the country there are exactly $n$ cities numbered with positive integers from $1$ to $n$ . In each city there is an airport is located.

Also, there is the only one airline, which makes $m$ flights. Unfortunately, to use them, you need to be a regular customer of this company, namely, you have the opportunity to enjoy flight $i$ from city $a_{i}$ to city $b_{i}$ only if you have already made at least $d_{i}$ flights before that.

Please note that flight $i$ flies exactly from city $a_{i}$ to city $b_{i}$ . It can not be used to fly from city $b_{i}$ to city $a_{i}$ . An interesting fact is that there may possibly be recreational flights with a beautiful view of the sky, which begin and end in the same city.

You need to get from city $1$ to city $n$ . Unfortunately, you've never traveled by plane before. What minimum number of flights you have to perform in order to get to city $n$ ?

Note that the same flight can be used multiple times.

The first line contains two integers, $n$ and $m$ ( $2<=n<=150$ , $1<=m<=150$ ) — the number of cities in the country and the number of flights the company provides.

Next $m$ lines contain numbers $a_{i}$ , $b_{i}$ , $d_{i}$ ( $1<=a_{i},b_{i}<=n$ , $0<=d_{i}<=10^{9}$ ), representing flight number $i$ from city $a_{i}$ to city $b_{i}$ , accessible to only the clients who have made at least $d_{i}$ flights.

Print "Impossible" (without the quotes), if it is impossible to get from city $1$ to city $n$ using the airways.

But if there is at least one way, print a single integer — the minimum number of flights you need to make to get to the destination point.