A16300. Line Trip

There is a road, which can be represented as a number line. You are located in the point $0$ of the number line, and you want to travel from the point $0$ to the point $x$ , and back to the point $0$ .

You travel by car, which spends $1$ liter of gasoline per $1$ unit of distance travelled. When you start at the point $0$ , your car is fully fueled (its gas tank contains the maximum possible amount of fuel).

There are $n$ gas stations, located in points $a_1, a_2, \dots, a_n$ . When you arrive at a gas station, you fully refuel your car. Note that you can refuel only at gas stations, and there are no gas stations in points $0$ and $x$ .

You have to calculate the minimum possible volume of the gas tank in your car (in liters) that will allow you to travel from the point $0$ to the point $x$ and back to the point $0$ .

The first line contains one integer $t$ ( $1 \le t \le 1000$ ) — the number of test cases.

Each test case consists of two lines:

- the first line contains two integers $n$ and $x$ ( $1 \le n \le 50$ ; $2 \le x \le 100$ );
- the second line contains $n$ integers $a_1, a_2, \dots, a_n$ ( $0 < a_1 < a_2 < \dots < a_n < x$ ).

For each test case, print one integer — the minimum possible volume of the gas tank in your car that will allow you to travel from the point $0$ to the point $x$ and back.

In the first test case of the example, if the car has a gas tank of $4$ liters, you can travel to $x$ and back as follows:

- travel to the point $1$ , then your car's gas tank contains $3$ liters of fuel;
- refuel at the point $1$ , then your car's gas tank contains $4$ liters of fuel;
- travel to the point $2$ , then your car's gas tank contains $3$ liters of fuel;
- refuel at the point $2$ , then your car's gas tank contains $4$ liters of fuel;
- travel to the point $5$ , then your car's gas tank contains $1$ liter of fuel;
- refuel at the point $5$ , then your car's gas tank contains $4$ liters of fuel;
- travel to the point $7$ , then your car's gas tank contains $2$ liters of fuel;
- travel to the point $5$ , then your car's gas tank contains $0$ liters of fuel;
- refuel at the point $5$ , then your car's gas tank contains $4$ liters of fuel;
- travel to the point $2$ , then your car's gas tank contains $1$ liter of fuel;
- refuel at the point $2$ , then your car's gas tank contains $4$ liters of fuel;
- travel to the point $1$ , then your car's gas tank contains $3$ liters of fuel;
- refuel at the point $1$ , then your car's gas tank contains $4$ liters of fuel;
- travel to the point $0$ , then your car's gas tank contains $3$ liters of fuel.