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>
#define rep(i,n)for(int i=0;i<(n);i++)
#define rev(s) (string((s).rbegin(), (s).rend()))
using namespace std;
typedef long long ll;
const long long INF = 1LL<<60;

int main() {
    int n;cin>>n;
    vector<int> b(n-1);
    rep(i,n-1) cin>>b[i];
    b.push_back(b[n-2]);
    vector<int> a(n);

    rep(i,n) {
        if (i==0) {
            a[i]=b[i];
        }
        else {
            a[i]=min(b[i-1],b[i]);
        }
    }
    ll ans = 0;
    rep(i,n) {
        ans+=a[i];
    }
    cout<<ans<<endl;
}