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 (1.15.1) (judge verdict: Accepted)

use std::io::Read;
use std::cmp::min;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();

    let answer = solve(&buf);

    println!("{}", answer);
}

fn solve(input: &str) -> String {
    let mut iterator = input.split_whitespace();

    let n: usize = iterator.next().unwrap().parse().unwrap();

    let mut a: Vec<usize> = vec![0; n];
    let b0: usize = iterator.next().unwrap().parse().unwrap();
    a[0] = b0;
    a[1] = b0;
    for i in 1..(n-1) {
        let bi: usize = iterator.next().unwrap().parse().unwrap();
        a[i + 1] = bi;
        a[i] = min(a[i], bi);
    }

    let answer = a.iter().fold(0, |acc, &x| acc + x);

    return answer.to_string();
}

#[test]
fn test() {
    assert_eq!(
        solve(
            r"6
0 153 10 10 23"
        ),
        "53"
    );
    assert_eq!(
        solve(
            r"2
3"
        ),
        "6"
    );
    assert_eq!(
        solve(
            r"3
2 5"
        ),
        "9"
    );
}