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 <iostream>
#include <vector>
using namespace std;

int main()
{
  int N;
  cin >> N;
  vector<int> B(N - 1);
  auto * const b = &B[0];
  for (int i = 0; i < N - 1; ++i)
    cin >> b[i];
  vector<int> A(N);
  auto * const a = &A[0];
  a[0] = b[0];
  a[N - 1] = b[N - 2];
  for (int i = 1; i < N - 1; ++i)
    a[i] = (b[i - 1] < b[i] ? b[i - 1] : b[i]);
  int sum = 0;
  for (int i = 0; i < N; ++i)
    sum += a[i];
  cout << sum;
  return 0;
}