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++ (GCC 9.2.1) (judge verdict: Accepted)

// https://atcoder.jp/contests/abc140/tasks/abc140_c
// 番兵(sentinel)を置くことでループの端を気にせずに処理する
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <vector>
#include <string>
#include <algorithm>
#include <numeric>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <set>
#include <unordered_set>
#include <bitset>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
#define REP(i, n) for(size_t i = 0; i < (n); ++i)
#define SENTINEL 1000000;
int main()
{
    int N;
    cin >> N;
    vector<int> B(N + 1);
    B[0] = B[N] = SENTINEL;
    for (int i = 1; i < N; ++i) cin >> B[i];
    int ans = 0;
    REP(i, N)
    {
        ans += min(B[i], B[i + 1]);
    }
    cout << ans << endl;
    return 0;
}