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 rep(i, a, n) for (int i = (int)(a); i <= (int)(n); ++i)
#define rrep(i, a, n) for (int i = (int)(a); i >= (int)(n); --i)
#define debug(x) cerr << #x << " = " << x << "\n"
#define debugv(x) \
  rep(f, 0, (x.size() - 1)) cerr << x[f] << (f == (x.size() - 1) ? "\n" : " ")
#define all(x) x.begin(), x.end()
#define int long long
using namespace std;
typedef pair<int, int> pii;
typedef vector<int> vi;
const int MX = 1e5 + 5, INF = 5 << 28, MOD = 1e9 + 7;

int N, M, K;
vector<int> A, B;
string S, T;

void input() {
  cin>>N;
  B.resize(N-1);
  rep(i,0,N-2){
    cin>>B[i];
  }
}
void solve() {
  int ans = 0;

  A.resize(N+2);

A[0] = B[0];
rep(i,1,N-1){
  A[i] = min(B[i-1],B[i]);
  if (i==N-1){
    A[i]=B[i-1];
  }
}
debugv(A);
for(auto x : A){
  ans += x;
}
  ;
  cout<<ans<<endl;
}
signed main() {
  input();
  solve();
  return 0;
}