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: Wrong Answer)

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using vec = vector<ll>;
using mat = vector<vec>;

#define INF (1LL << 60)
#define MOD 1000000007

#define REP(i,m,n) for(ll (i)=(m),i_len=(n);(i)<(i_len);++(i))
#define FORR(i,v) for(auto (i):v)
#define ALL(x) (x).begin(), (x).end()
#define PR(x) cout << (x) << endl
#define PS(x) cout << (x) << " "
#define SZ(x) ((ll)(x).size())
#define MAX(a,b) (((a)>(b))?(a):(b))
#define MIN(a,b) (((a)<(b))?(a):(b))
#define pb push_back
#define mp make_pair

int main()
{
    ll N;
    cin >> N;
    vec B(N-1);
    REP(i,0,N-1) cin >> B[i];

    ll sum = B[0];
    REP(i,1,N) sum += MIN(B[i],B[i-1]);
    PR(sum);

    return 0;
}

/*

  B[i] >= max(A[i], A[i+1])
  <=> B[i] >= A[i] and B[i] >= A[i+1]
  
  A[0] <= B[0]
  A[i] <= B[i] and A[i] <= B[i-1] (i = 1, ..., N-1)
  
  A[0] = B[0]
  A[i] = MIN(B[i],B[i-1]) (i = 1, ..., N-1)

*/