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 Python (3.4.3) (judge verdict: Wrong Answer)

n = int(input())
# a: 長さnの値不明の整数列
b = [int(i) for i in input().split()]

# b1 >= max(a1,a2)
# b2 >= max(a2,a3)
# ...
# bn-1 >= max(an_1,an)

# この条件を満たす最大のa1,a2,...を求める
# sum_i{bi}がなにか使えそうなきはする 

# [ex1]
# b = [2,5,5]
# a = [2,2,5,1]
#     [2,2,5,5] *

# [ex2]
# b = [0,153,10,10,23]
# a = [0,0,153,10,10,23] ... false!
#     [0,0, 10,10,10,23] ... true
#
# bi > bi+1 のとき、bi = bi+1にする必要がある

# a1 = a2 = b1
# a3 = b2
# ...
# an = bn-1 

# よって、ans = b1 + sum_i{bi}
b2 = [] 
for i in range(len(b)-1):
    if b[i] > b[i+1]:
        b2.append(b[i+1])
    else:
        b2.append(b[i])
b2.append(b[-1])

ans = b2[0] + sum(b2)
print(ans)