Chickens and Rabbits (雞兔同籠)

1. Suppose you raise chickens and rabbits together in a cage. As we know, each chicken has two legs, while each rabbit has four legs. If you only know the total number of animals, and the total number of legs, can you write a program to calculate the exact number of chickens and the number of rabbits?
2. Input: number of animals (a) & number of legs (leg).
3. Output: number of chickens & number of rabbits.
4. You may derive a special formula to solve this problem, or you may also re-use the program you developed in the homework of solving linear equations.
5. Note: in this exercise, let us assume that the given input will always yield a valid solution.