Distances Between Leaf Vertices

A rooted binary tree has n leaf vertices. A leaf vertex is a vertex with no children. When the tree is traversed inorder, the leaf vertices are numbered 1, 2, ..., n in the order they appear. Every vertex that has children has exactly two children.

For each integer k with 1 <= k < n, the distance between leaf vertex k and leaf vertex k + 1 is known. This information is enough to determine the distance between any two leaf vertices. Given two distinct leaf vertex numbers, compute their distance.

The first line contains the number of leaf vertices n (2 <= n <= 1,000).

The next n - 1 lines contain, in order, the distance between leaf vertices 1 and 2, the distance between 2 and 3, ..., and the distance between n - 1 and n.

The next line contains two distinct leaf vertex numbers whose distance must be found.

Print the distance on the first line.