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)

#include<iostream>
#include<iomanip>
#include<string>
#include<algorithm>
#include<vector>
#include<utility>
#include<tuple>
#include<map>
#include<queue>
#include<stack>
#include<deque>
#include<bitset>
#include<math.h>
using namespace std;
using ll = int64_t;
using Graph = vector<vector<int> >;
const ll M = 1000000007;

int main(){
    int n;
    cin >> n;
    int b[n-1];
    for(int i=0;i<n-1;i++) cin >> b[i];

    int ans=b[0];
    for(int i=0;i<n-2;i++) ans+=min(b[i],b[i+1]);
    ans+=b[n-2];

    cout << ans << endl;
}