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)

fn read<T: std::str::FromStr>() -> T {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
	read::<String>().split_whitespace()
		.map(|e| e.parse().ok().unwrap()).collect()
}

fn read_vec2<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
	(0..n).map(|_| read_vec()).collect()
}

pub trait LexicalPermutation {
	/// Return `true` if the slice was permuted, `false` if it is already
	/// at the last ordered permutation.
	fn next_permutation(&mut self) -> bool;
}
impl<T> LexicalPermutation for [T] where T: Ord {
	/// Original author in Rust: Thomas Backman <serenity@exscape.org>
	fn next_permutation(&mut self) -> bool {
		// These cases only have 1 permutation each, so we can't do anything.
		if self.len() < 2 { return false; }

		// Step 1: Identify the longest, rightmost weakly decreasing part of the vector
		let mut i = self.len() - 1;
		while i > 0 && self[i-1] >= self[i] {
			i -= 1;
		}

		// If that is the entire vector, this is the last-ordered permutation.
		if i == 0 {
			return false;
		}

		// Step 2: Find the rightmost element larger than the pivot (i-1)
		let mut j = self.len() - 1;
		while j >= i && self[j] <= self[i-1]  {
			j -= 1;
		}

		// Step 3: Swap that element with the pivot
		self.swap(j, i-1);

		// Step 4: Reverse the (previously) weakly decreasing part
		self[i..].reverse();

		true
	}
}

fn main() {
	let n: usize = read();
	let mut b: Vec<usize> = read_vec();

	let mut a: Vec<usize> = vec![0; n];
	a[0] = b[0];
	a[1] = b[0];
	for i in 1..n-1 {
		let bi = b[i];
		a[i + 1] = bi;
		a[i] = std::cmp::min(a[i], bi);
	}

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