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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;



class Program
{
    const int MOD = 1000000007;
    const int INF = 1 << 30;



    static void Main(string[] args)
    {
        Solve();
    }



    static void Solve()
    {
        Scaner sc = new Scaner();
        write wr = new write();
        Math2 mmath = new Math2();
        //ここから

        var xx=sc.LongArr;
        var N=xx[0];
        var B=sc.LongArr;

        var A=new long[N];
        A[0]=B[0];
        for(int i=1;i<N;++i){
            if(i!=N-1)
            A[i]=Math.Min(B[i-1],B[i]);
            else A[i]=B[i-1];
        }
        long ans=0;
        for(int i=0;i<N;++i){
            ans+=A[i];
        }
wr.wri(ans);
    }


    
}

class PriorityQueue<T>
{
    private readonly List<T> heap;
    private readonly Comparison<T> compare;
    private int size;
    public PriorityQueue() : this(Comparer<T>.Default) {} 
    public PriorityQueue(IComparer<T> comparer) : this(16, comparer.Compare) { }
    public PriorityQueue(Comparison<T> comparison) : this(16, comparison) { }
    public PriorityQueue(int capacity, Comparison<T> comparison)
    {
        this.heap = new List<T>(capacity);
        this.compare = comparison;
    }
    public void Enqueue(T item)
    {
        this.heap.Add(item);
        var i = size++;
        while (i > 0)
        {
            var p = (i - 1) >> 1;
            if (compare(this.heap[p], item) <= 0)
                break;
            this.heap[i] = heap[p];
            i = p;
        }
        this.heap[i] = item;

    }
    public T Dequeue()
    {
        var ret = this.heap[0];
        var x = this.heap[--size];
        var i = 0;
        while ((i << 1) + 1 < size)
        {
            var a = (i << 1) + 1;
            var b = (i << 1) + 2;
            if (b < size && compare(heap[b], heap[a]) < 0) a = b;
            if (compare(heap[a], x) >= 0)
                break;
            heap[i] = heap[a];
            i = a;
        }
        heap[i] = x;
        heap.RemoveAt(size);
        return ret;
    }
    public T Peek() { return heap[0]; }
    public int Count { get { return size; } }
    public bool Any() { return size > 0; }
}


 class Math2
{
    public long[] Fact;  //Nまでの階乗
    public long[] invFact;  //階乗の逆元
    public void FactoricalModSet(long N,long MOD=1000000007)
    {
        Fact = new long[N + 1];
        invFact = new long[N + 1];

        Fact[0] = 1;
        for(long i = 1; i <= N; ++i)
        {
            Fact[i] = Fact[i - 1] * i%MOD;
        }
        invFact[N] = powMOD(Fact[N], MOD - 2);

        for (long i = N; i > 0; --i)
        {
            invFact[i - 1] = invFact[i] * i%MOD;
        }

    }

    public long nCrMod(long n,long r,long MOD=1000000007)
    {
        return (Fact[n] * invFact[r] % MOD * invFact[n-r]%MOD);
    }

    public long powMOD(long x,long n,long MOD = 1000000007)
    {
        long res = 1;
        while (n > 0)
        {
            if ((n & 1) != 0) res = res * x % MOD;
            x = x * x % MOD;
            n >>= 1;
        }
        return res;
    }

    static long GCD(long x, long y)
    {
        if (y == 0) return x;
        return GCD(y, x % y);
    }

    static long LCM(long x, long y)
    {
        return x * y / GCD(x, y);
    }
}


class Scaner
{
    public string Str => Console.ReadLine();

    public string[] StrArr => Str.Split(' ');

    public long[] LongArr => StrArr.Select(long.Parse).ToArray();
    public int[] IntArr => StrArr.Select(int.Parse).ToArray();
    public char[] CharArr => Str.ToArray();
    public long Long1 => LongArr[0];
    public int Int1 => IntArr[0];
    
}



class write
{
    public  void wri<T>(T x)
    {
        Console.WriteLine(x);
    }
}