You are given 2 qubits which are guaranteed to be in one of the four orthogonal states:

$\text{[math]}$
$\text{[math]}$
$\text{[math]}$
$\text{[math]}$

Your task is to perform necessary operations and measurements to figure out which state it was and to return the index of that state (0 for $\text{[math]}$ , 1 for $\text{[math]}$ etc.). The state of the qubits after the operations does not matter.

You have to implement an operation which takes an array of 2 qubits as an input and returns an integer.

Your code should have the following signature:

namespace Solution {
    open Microsoft.Quantum.Primitive;
    open Microsoft.Quantum.Canon;

    operation Solve (qs : Qubit[]) : Int
    {
        body
        {
            // your code here
        }
    }
}

Distinguish four 2-qubit states