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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;
 
public class Main {
    public static void main(String[] args) throws Exception {
        //入力
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
      	//Bの配列格納
        int[] B = new int[N-1];
        for (int i = 0; i < (N-1); ++i){
            B[i]= sc.nextInt();
        }
        
        int sum = B[0] + B[N-2];
      	
        for (int i = 0; i < (N-2); ++i){
          	//比較して小さい方を加算
            sum += Math.min(B[i],B[i+1]);
        }
        
        //出力
        System.out.println(sum);
        
    }
}