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)

/*
g++ -std=c++14 -O2 -Wall a.cpp -o a && a
*/

//  io
#include <iostream>
//#include <fstream> //http://www.cplusplus.com/reference/fstream/fstream/
//#include <cstdio>
#include <iomanip> //http://www.cplusplus.com/reference/iomanip/
#include <limits>
//  util
#include <string>
#include <cstring>
#include <algorithm>
//#include <utility>
//#include <functional> //http://www.cplusplus.com/reference/functional/
//#include <cstdlib>
#include <cmath>
//#include <complex>
//#include <random>
//#include <bitset>
//  containers https://en.cppreference.com/w/cpp/container
#include <vector>
//#include <queue>
#include <set>
//#include <map>
//#include <unordered_set>
//#include <unordered_map>
//  time
//#include <ctime>
//#include <chrono>
//                  https://en.cppreference.com/w/cpp/header/chrono
//                  http://www.cplusplus.com/reference/chrono/

using namespace std;
typedef vector<int> arri;
typedef vector<float> arrf;
typedef pair<int, int> pairi;
typedef long long ll;
typedef long double ld;
const int mod = 1e9 + 7;
const ll inf = 0x7fffffffffffffff;
const ll infneg = -0x7fffffffffffffff;

//file input routed to stdinout
/*
std::ifstream inFile("input.txt");
std::ofstream outFile("output.txt");
std::cin.rdbuf(inFile.rdbuf());
std::cout.rdbuf(outFile.rdbuf());
*/

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    cin >> n;

    arri a(n);
    arri b(n);

    for (int i = 0; i < n - 1; i++)
    {
        cin >> b[i];
    }

    int all = b[n - 2];
    a[n - 1] = all;

    for (int i = n - 2; i > 0; i--)
    {
        all = min(all, b[i - 1]);
        a[i] = all;
    }
    all = min(all, b[0]);
    a[0] = all;

    ll sum = 0;
    for (int i = 0; i < n; i++)
    {
        sum += a[i];
    }

    cout << sum;
}