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;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define endl '\n';
#define FASTER ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
const ll POSITIVE_INFINITY = 9223372036854775807;
const ll NEGATIVE_INFINITY = -9223372036854775807;
const ll MOD = 1000000007;
const ld PI = acos(-1.0);

template <typename T> inline T bigMod(T A,T B,T M = MOD){A %= M;T ret = 1;while(B){if(B & 1) ret = (ret * A) % M;A = (A * A) % M;B = B >> 1;}return ret;}
template <typename T> inline T invMod (T A,T M = MOD){return bigMod(A,M-2,M);}

const int N = 200005;

int main(){
  FASTER;
  int n;
  cin >> n;
  vector<ll>ar(n);
  for (int i = 1; i < n; i++){
    cin >> ar[i];
  }
  ar[0] = ar[1];
  for (int i = 0; i < n - 1; i++){
    ar[i] = min(ar[i], ar[i + 1]);
  }
  ll sum = 0;
  for (int i = 0; i < n; i++){
    sum += ar[i];
  }
  cout << sum << '\n';
  return 0;
}