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++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define rep(i, n)	for(int i=0;i<(int)(n);i++)
using namespace std;
using ll=long long;

int main(){
    int n;
    cin>>n;
    int b[100];
    int ans=0;
    rep(i,n-1) {
        cin>>b[i];
    }
    vector<int> c(n,-1);
    rep(i,n-1){
        if(i!=n-2){
            if(b[i]<=b[i+1]){
                //cout<<c[i]<<endl;
                if(c[i]==-1) c[i]=b[i];
                c[i+1]=b[i];
                //cout<<1<<endl;
            }else{
                //cout<<c[i]<<endl;
                if(c[i]==-1) c[i]=b[i];
                //cout<<2<<endl;
            }
        }else{
            if(c[i]==-1) c[i]=b[i];
            c[i+1]=b[i];
        }
        //cout<<c[i]<<endl;
    }
    rep(i,n) {
        ans+=c[i];
        //cout<<ans<<endl;
    }
    cout<<ans<<endl;
}