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>
#include <iostream>
#include <math.h>
using namespace std;
 
int main() {
  int a,N;
  a = 0;
  cin >> N;
  vector<int> A(N);
  vector<int> B(N-1);
  for (int i=0;i<N-1;i++){
  cin >> B.at(i) ;
  }
  A.at(0) = B.at(0);
  for (int j = 0;j<N-2;j++){
    A.at(j+1) = min(B.at(j),B.at(j+1));
  }
  A.at(N-1) = B.at(N-2);
  for (int k = 0;k<N;k++){
  a += A.at(k);
  }
  cout << a << endl;
}