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 lli long long int
#define MODV 1000000007
#define INF INT_MAX // 2147483647
#define INFLL LLONG_MAX // 9223372036854775807
#define rep(i, n) for(lli i=0, i##_len=(lli)(n); i<i##_len; i++)
#define repf(i, n) for(lli i=1, i##_len=(lli)(n+1); i<i##_len; i++)
#define per(i, n) for(lli i=((lli)(n))-1; i>=0; i--)
#define perf(i, n) for(lli i=((lli)(n)); i>0; i--)
#define all(v) v.begin(), v.end()
#define endl "\n"
#define vi vector<lli>
#define vvi vector<vector<lli>>
#define Yes() cout << "Yes" << endl
#define YES() cout << "YES" << endl
#define No() cout << "No" << endl
#define NO() cout << "NO" << endl
template<class T>bool chmax(T &a, const T &b){ if(a<b){ a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b){ if(b<a){ a=b; return 1; } return 0; }
using namespace std;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(10);
    lli n;
    cin >> n;
    vi a(n, 0);
    vi b(n, INFLL);
    rep(i, n-1) cin >> b[i];
    a[0] = b[0];
    repf(i, n-1) a[i] = min(b[i-1], b[i]);
    lli sum = 0;
    rep(i, n) sum += a[i];
    cout << sum << endl;
}