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 ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
using namespace std;
using ll = long long;
const ll MOD = 1000000007;
const ll INF = MOD * MOD;
const int inf = (1<<29);

int main() {

    int n;
    cin >> n;

    vector<int> a(n);
    REP(i, n-1) cin >> a[i];
    
    int ans = a[0];    
    for(int i = 1; i < n-1; i++) {
        ans += min(a[i-1], a[i]);
    }

    ans = ans + a[n-2];

    cout << ans << endl;
    
    return 0;
}