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**Task** | Statistics

Time limit: | 1.00 s | Memory limit: | 512 MB |

You play a game consisting of $n$ rooms and $m$ tunnels. Your initial score is $0$, and each tunnel increases your score by $x$ where $x$ may be both positive or negative. You may go through a tunnel several times.

Your task is to walk from room $1$ to room $n$. What is the maximum score you can get?

The first input line has two integers $n$ and $m$: the number of rooms and tunnels. The rooms are numbered $1,2,\dots,n$.

Then, there are $m$ lines describing the tunnels. Each line has three integers $a$, $b$ and $x$: the tunnel starts at room $a$, ends at room $b$, and it increases your score by $x$. All tunnels are one-way tunnels.

You can assume that it is possible to get from room $1$ to room $n$.

Print one integer: the maximum score you can get. However, if you can get an arbitrarily large score, print $-1$.

- $1 \le 2500 \le n$

- $1 \le 5000 \le m$

- $1 \le a,b \le n$

- $-10^9 \le x \le 10^9$

Input:

`4 5`

1 2 3

2 4 -1

1 3 -2

3 4 7

1 4 4

Output:

`5`