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 <stdio.h>
#include <iostream>
#include <vector>
#include <cstdint>
#include <algorithm>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main(){
int N; cin >> N;
vector<int> B(N-1);
vector<int> A(N);
rep(i,N-1){
  cin >> B[i];
}
A[0] = B[0];
A[N-1] = B[N-2];
for(int i = 1; i < N - 1; i++){
  if(B[i-1] > B[i]){
    A[i] = B[i];
  }
  else{
    A[i] = B[i-1];
  }
}
int sum = 0;
  for(int j = 0; j < N; j++){
    sum+= A[j];
    //cout << A[j] << endl;
  }
  cout << sum << endl;
}