CF1901A 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.