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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
#define PB push_back

int main() {
	ios_base::sync_with_stdio(false); cin.tie(0);
	int n; cin >> n;
	int a[n], b[n-1];
	for (int i = 0; i < n - 1; ++i) cin >> b[i];
	for (int i = n - 2; i >= 0; --i) {
		a[i] = b[i];
		if (i == n - 2) a[i + 1] = b[i];
		else a[i + 1] = min(a[i+1], b[i]);
	}
	int res = 0;
	for (int i = 0; i < n; ++i) {
		res += a[i];
	} 
	cout << res;
}