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;

fn main() {
    let mut buf = String::new();

    std::io::stdin().read_to_string(&mut buf).unwrap();

    let mut iter = buf.split_whitespace();
    let n: usize = iter.next().unwrap().parse().unwrap();
    let mut b: Vec<usize> = vec![];
    for _ in 0..(n - 1) {
        b.push(iter.next().unwrap().parse().unwrap());
    }

    let mut a: Vec<usize> = vec![];

    for i in 0..n {
        if i == 0 {
            a.push(b[0]);
            continue;
        } else if i == n - 1 {
            a.push(b[i - 1]);
            break;
        }
        a.push(std::cmp::min(b[i], b[i - 1]));
    }

    let mut ans = 0;
    for a in a.into_iter() {
        ans += a;
    }

    // let sum = a.iter().fold(0, |sum, a| sum + a.count);

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