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>
#include <algorithm>

int main()
{
    int N;
    std::cin >> N;
    std::vector<int> A(N), B(N-1);
    for(int i=0; i<N-1; i++) scanf(" %d", &B[i]);

    int sum = 0;
    A[N-1] = B[N-2];
    for(int i=N-2; i>=1; i--){
        A[i] = std::min(B[i], B[i-1]);
    }
    A[0] = B[0];

    for(int i=0; i<N; i++) sum += A[i];
    std::cout << sum << std::endl;
}