C# assistance with parsing my own expression tree

I have the code shown below. I created an expression tree, but I'm having trouble parsing it to get the outcome.

The specifics are included in my code.

public enum OpertaionType { add, sub, div, mul} 

public class Node {
    public Node(Node lhs, Node rhs, OpertaionType opType, int index) {
        this.lhs = lhs;
        this.rhs = rhs;
        this.opType = opType;
        this.index = index;
    }
    Node lhs;
    Node rhs;
    OpertaionType opType;
    int index;
}
class Program
{
    static void Main(string[] args)
    {
        // I don't want this to be part of the node data structure
        // because in the actual implementation I will end up referencing an
        // array of data

        int[] value = new int[5];

        Node[] nodes = new Node[value.Length];
        for (int i = 0; i < value.Length; i++)
        {
            value[i] = i+1;
            nodes[i] = new Node(null, null, 0, i);
        }



        // suppose I constructed the tree like that 
        // note that the end nodes are marked by non-negative index that indexes the
        // values array

        Node a = new Node(nodes[0], nodes[1], OpertaionType.add, -1);
        Node b = new Node(nodes[2], a, OpertaionType.mul, -1);
        Node c = new Node(b, nodes[3], OpertaionType.div, -1);

        // How can I find the result of Node c programatically
        // such that the result is (a[2]*(a[0]+a[1]))/a[3] = 9/4

    }
}