Simple calculations

There is a sequence of n+2 elements a0,a1,...,an+1 (n ≤ 3000;−1000 ≤ ai ≤ 1000). It is known that ai = ai−1 +ai+1 −ci 2 for each i = 1,2,...,n. You are given a0,an+1,c1,...,cn. Write a program which calculates a1. Input The first line is the number of test cases, followed by a blank line. For each test case, the first line of the input contains an integer n. The next two lines consist of numbers a0 and an+1 each having two digits after decimal point, and the next n lines contain numbers ci (also with two digits after decimal point), one number per line. Each test case will be separated by a single line. Output For each test case, the output should contain a1 in the same format as a0 and an+1. Print a blank line between the outputs for two consecutive test cases. Sample Input 1 1 50.50 25.50 10.15 Sample Output 27.85