Problem:
Score : 300 points

Problem Statement

There is an integer sequence A of length N whose values are unknown.

Given is an integer sequence B of length N-1 which is known to satisfy the following:

B_i \geq \max(A_i, A_{i+1})

Find the maximum possible sum of the elements of A.

Constraints

All values in input are integers.

2 \leq N \leq 100

0 \leq B_i \leq 10^5

Input

Input is given from Standard Input in the following format:

N
B_1 B_2 ... B_{N-1}

Output

Print the maximum possible sum of the elements of A.

Sample Input 1

3
2 5

Sample Output 1

9

A can be, for example, ( 2 , 1 , 5 ), ( -1 , -2 , -3 ), or ( 2 , 2 , 5 ). Among those candidates, A = ( 2 , 2 , 5 ) has the maximum possible sum.

Sample Input 2

2
3

Sample Output 2

6

Sample Input 3

6
0 153 10 10 23

Sample Output 3

53

Solution in OCaml (4.10.0) (judge verdict: Accepted)

let () =
  Scanf.scanf "%d\n" @@ fun n ->
  let blst = Array.to_list
      (Array.init (n-1) @@ fun _ -> Scanf.scanf "%d " @@ fun d -> d) in
  let (_,ans) = List.fold_left
      (fun (bfr,ans) x -> (x,ans + (min bfr x)))
      (List.hd blst, 0) blst in
  Printf.printf "%d\n" (ans + (List.hd (List.rev blst)))