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;
int main(){
  int N;
  cin>>N;
  vector<int>B(N-1);
  vector<int>A(N,100000);
  for(int i=0;i<N-1;i++){
    cin>>B.at(i);
  }
  for(int i=0;i<N-1;i++){
    A.at(i)=min(A.at(i),B.at(i));
    A.at(i+1)=min(A.at(i+1),B.at(i));
  }
  long long sum=0;
  for(int i=0;i<N;i++){
    sum=sum+A.at(i);
  }
  cout<<sum<<endl;
}