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 Lua (5.3.2) (judge verdict: Accepted)

local read = setmetatable({}, {__index = function(t, k) local a = {} for i=1,#k do table.insert(a, string.sub(k, i, i)) end local r = io.read local u = table.unpack return function() return r(u(a)) end end})
local N = read.n()
local B = {}
for i=1,N-1 do B[i] = read.n() end

local A = {}
A[N] = B[N-1]
local ans = A[N]
for i=N-1,2,-1 do
    A[i] = math.min(B[i-1], B[i])
    ans = ans + A[i]
end
A[1] = B[1]
ans = ans + A[1]
--print(table.concat(A, "\t"))
--print(table.concat(B, "\t"))
print(ans)