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 Rust (1.42.0) (judge verdict: Accepted)

#![allow(non_snake_case)]
#![allow(unused_imports)]

use proconio::{input, fastout};
use proconio::marker::*;
use whiteread::parse_line;
use std::collections::*;
use num::*;

use itertools::Itertools;
use itertools_num::ItertoolsNum;

#[fastout]
fn main() {
    input!{
        n: usize,
        b_vec: [usize; n-1],
    }

    let mut a_vec: Vec<usize> = vec![0; n];

    for i in 0..n-1 {
        if i == 0 {
            a_vec[i] = b_vec[i];
            a_vec[i+1] = b_vec[i];
        } else {
            if b_vec[i] > b_vec[i-1] {
                a_vec[i+1] = b_vec[i]
            } else {
                a_vec[i+1] = b_vec[i];
                a_vec[i] = b_vec[i];
            }
        }
    }

    println!("{}", a_vec.iter().sum::<usize>());
}