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>
using namespace std;
#define FOR(i,a,b) for (int i = a; i < b; i++)
#define REP(i,b) FOR(i,0,b)
#define RFOR(i,a,b) for (int i = a-1; i >= b; i--)
#define RREP(i,a) RFOR(i,a,0)
#define REPALL(i,v) for (int i = 0; i < v.size(); i++)
#define RREPALL(i,v) for (int i = v.size()-1; i >= 0; i--)
#define ALL(v) v.begin(), v.end()
#define SORT(v) sort(v.begin(), v.end())
#define MIN_ELEMENT(v) min_element(v.begin(), v.end())
#define MAX_ELEMENT(v) max_element(v.begin(), v.end())
#define COUNT(v,n) count(v.begin(), v.end(), n)
void YES(bool flag) {cout<<(flag ? "YES" : "NO")<<endl;}
void Yes(bool flag) {cout<<(flag ? "Yes" : "No")<<endl;}
void yes(bool flag) {cout<<(flag ? "yes" : "no")<<endl;}
typedef long long ll;
typedef unsigned long long ull;
const int INF = 1e7;
const ll MOD = 1e9 + 7;

vector<int> a, b;

int main()
{
    int n;
    cin>>n;
    b.resize(n-1);
    REP(i,n-1) cin>>b[i];
    b.push_back(INF);

    //REPALL(i,b) cout<<b[i]<<" ";

    int ans = b[0];
    REP(i,n) {
        ans += min(b[i], b[i+1]);
    }
    cout<<ans<<endl;

    return 0;
}