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)
using namespace std;
typedef long long ll;

int main(){
	int n;
	cin >> n;
	vector<int> b(n-1);
	rep(i,n-1) cin >> b[i];
	/*
	vector<int> a(n);
	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];
	*/
	
	int res = 0;
	res += b[0];
	for(int i = 1; i < n-1; i++) res += min(b[i-1], b[i]);
	res += b[n-2];
	cout << res << endl;
	return 0;
}