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: Wrong Answer)

#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <bits/stdc++.h>
#include <iomanip>
#include <numeric>
#include <utility>
#include <map>
#include <algorithm>
using namespace std;

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    long long N;
    cin >> N;
    vector<long long> vc(N);
    for (int i = 0; i < N - 1; ++i)
        cin >> vc[i];
    vc[N - 1] = vc[N - 2];
    long long sum = 0;
    vector<long long> vec(N);
    vec[0] = vc[0];
    vec[1] = vc[0];
    for (int i = 2; i < N; ++i)
        vec[i] = min(vc[i], vc[i + 1]);
    cout << "\n";
    cout << accumulate(vec.begin(), vec.end(), 0) << "\n";
}