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)

use std::io::*;
use std::str::*;

fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let s = stdin
        .bytes()
        .map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect::<String>();
    s.parse::<T>().ok().unwrap()
}

fn usize() -> usize {
    read()
}

fn vec_usize(n: usize) -> Vec<usize> {
    let mut vec = Vec::new();
    for _ in 0..n {
        vec.push(usize())
    }
    vec
}

fn main() {
    let n = usize();
    let bs = vec_usize(n - 1);
    let mut a = vec![0; n];

    a[0] = bs[0];
    for i in 1..n - 1 {
        a[i] = bs[i - 1].min(bs[i]);
    }
    a[n - 1] = bs[n - 2];
    let sum: usize = a.into_iter().sum();
    println!("{}", sum);
}