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 <numeric>

using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, x) for(int i=1;i<=(int)(x);i++)
#define all(x) (x).begin(),(x).end()

#include <bits/stdc++.h>

using namespace std;

int main() {
    int N;
    cin >> N;

    vector<int> A(N);
    vector<int> B(N - 1);

    rep(i, N - 1)cin >> B.at(i);

    A.at(0) = B.at(0);

    for (int i = 1; i <= N - 2; i++) {
        A.at(i) = min(B.at(i), B.at(i - 1));
    }

    A.at(N - 1) = B.at(N - 2);

    int ans =0;
    rep(i,N) ans+=A.at(i);
    cout << ans << endl;


}