C - Maximal Value

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 $A$.

3
2 5

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.

2
3

6

6
0 153 10 10 23

53