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>
using namespace std;
int b[105],a[105];
int main(){
    int n,sum=0;
    scanf("%d",&n);
    for(int i=1;i<n;i++){
        scanf("%d",&b[i]);
    }
    a[1]=a[2]=b[1];
    for(int i=2;i<n;i++){
        if(b[i]<a[i]) a[i]=b[i];
        a[i+1]=b[i];
    }
    for(int i=1;i<=n;i++){
        sum+=a[i];
    }
    printf("%d\n",sum);
}