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;
using ll = long long;

int main()
{
    int N;
    cin >> N;
    ll B[N];
    for(int i = 0; i < N-1; ++i) cin >> B[i];

    ll A[N+1];
    A[0] = B[0];
    for(int i = 1; i < N - 1; ++i) {
        A[i] = min(B[i-1], B[i]);
    }
    A[N-1] = B[N-2];
    ll ans = 0;
    for(int i = 0; i < N; ++i) ans += A[i];
    cout << ans << endl;
    return 0;
}