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 <iostream>
#include <vector>
#include <array>
#include <list>
#include <queue>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <string>
#include <sstream>
#include <algorithm>
#include <random>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <cmath>
#include <cassert>
#include <climits>
#include <bitset>
#include <functional>
#include <iomanip>

#define FOR_LT(i, beg, end) for (int i = beg; i < end; i++)
#define FOR_LE(i, beg, end) for (int i = beg; i <= end; i++)
#define FOR_DW(i, beg, end) for (int i = beg; end <= i; i--)

using namespace std;

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	int n; cin >> n;
	vector<int> cs(n - 1);
	for (auto& c : cs) {
		cin >> c;
	}

	int sum = cs[0];

	FOR_LT(i, 0, n - 2) {
		sum += min(cs[i], cs[i + 1]);
	}
	sum += cs[n - 2];

	cout << sum << endl;

	return 0;
}