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: Wrong Answer)


#include <iostream>
#include <string>
#include <algorithm>
#include <set>
#include <cmath>
#include <vector>

typedef long long ll;
const ll LL_MAX (1LL<<60);

#define rep(i,s,e) for(ll i=(s); i<(e); i++) 

using namespace std;

int main()
{
    ll n;
    cin >> n;

    vector<ll> b(n);

    rep(i, 0, n-1)
        cin >> b[i];

    vector<ll> a(n+1, 0);

    a[0] = b[0];
    rep(i, 0, n)
    {
        a[i] = b[i];

        if (b[i] < b[i+1])
        {
            a[i+1] = b[i];
            i++;
        }
    }
    a[n-1] = b[n-2];

    ll sum (0);
    rep(i, 0, n)
    {
        //cout << a[i] << endl;
        sum += a[i];
    }

    cout << sum << endl;

    return 0;
}