Applying a permutation on a sequence with multiplication

Question

We are given a sequence of $n$ numbers called $\alpha$ and an arbitrary number $x$. Give an algorithm to find a permutation $\pi$ of size $n$ such that $\sum_{i=1}^n{\alpha_i.\pi_i} = x$ or tell if there is none.

Well I wasn't able to solve it in polynomial time and I'm not sure whether it's possible. So if it's NP would you give me some observations on the reason?