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 <bits/stdc++.h>
#include <math.h>
#define say(a) cout<<a<<endl
#define say1(a,b) cout<<a<<" "<<b<<endl
#define loop(a,b) for(int i=a;i<b;i++)
#define pi 2*acos(0.0)
#define ll long long
#define f first
#define s second
//vector<int> v[200009];
using namespace std;
int mod =1e9;
int main()
{
    int n,sum=0,i;
    cin>>n;
    int a[n-1];
    for(i=0;i<(n-1);i++)
    {
        scanf("%d",&a[i]);
    }if(n==1) sum=2*a[0];
    else
    {
        sum+=a[n-2];
        for(i=(n-2);i>=0;i--)
    {
        if(i==0)
        {
            sum+=a[i];
        }else
        {
            sum+=min(a[i],a[i-1]);
        }
    }}say(sum);
}