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 ll long long
#define REP(i,n) for(long long i=0;i<n;++i)
#define REPP(i,m,n) for(long long i=m;i<n;++i)
#define rep(i,n) for(long long i = n-1;i>=0;--i)
#define repp(i,n,m) for(long long i = n-1; i >= m; --i)
#define ALL(N) (N.begin(),N.end())
#define de cout << "line : " << __LINE__ << " debug" << endl;
#define pb push_back
#define pq priority_queue
#define Dcout(N) cout << setprecision(20) << N << endl
constexpr int MOD = 1000000007;
constexpr int INF = 2147483647;
constexpr long long INFF = 9223372036854775807;

signed main() {
    cin.tie(0);
	ios::sync_with_stdio(false);
    int N;cin >> N;
    int B[N-1];REP(i,N-1) cin >> B[i];
    if(N == 2){
        cout << B[0]*2 << endl;
    }
    else{
        int sum = 0;
        REP(i,N-2){
            sum += min(B[i],B[i+1]);
        }
        cout << sum + B[0] + B[N-2] << endl;
    }
}