Accoing to Goldpach Conjecture, an even can be seperated into 2 primes, and others bigger than 2 can be seperated into 3.
So if the input is 1, output -1. If it is 2, output 1. If it is even other than 2, ouput 2.
For odd, if it is 2+prime, output 2, else output 3.
Is there anything I missed? Could anyone tell me why I got WA with above?