type 'a bt = Empty | Node of 'a * 'a bt * 'a bt;;

let tt = Node(1,Node(2,Node(4,Empty,Empty),Empty),Node(3,Node(5,Empty,Node(6,Empty,Empty)),Empty));;

let internalPath tree=
    let rec helper Node(value,tl,tr) depth=
        match Node with
        | Empty -> 0
        | Node(_,tl,tr) -> depth + helper(depth+1) tl + helper(depth+1) tr
        in helper 0 tree;;
internalPath tt;;