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 <sstream>
#include <vector>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iomanip>
#include <vector>
#include <cstdio>
#include <float.h>
#include <queue>
#include <set>
#include <map>
#include <fstream>
#include <cstdlib>
#include <string>
#include <cstring>
#include <algorithm>
#include <numeric>
#include <stack>
#include <functional>
 
using namespace std;
 
#define endl '\n'
#define MOD 1000000007
#define INF 1ll<<30
#define MAX 100010
#define eps 1e-11
#define bit_max 1ll<<32
#define _USE_MATH_DEFINES
 
 
 
 
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
  //  freopen("./input.txt","r",stdin);
 //   freopen("./output.txt","w",stdout);
    long long int n;
    cin>>n;
    int b[n];
    for(int i=0;i<n-1;i++)
        cin>>b[i];
    int a[n]={0};
    for(int i=0;i<n;i++)
    {
        if(i==0)
        {
            a[i] = b[i];
        }
        else if(i==n-1)
        {
            a[i]= b[i-1];
        }
        else
        {
            a[i]=min(b[i],b[i-1]);
        }
    }
    long long int ans =0;
    for(int i=0;i<n;i++)
    {
        ans+=a[i];
    }
    cout<<ans;
}