Problem:
Score : 300 points

Problem Statement

There are N+1 towns. The i-th town is being attacked by A_i monsters.

We have N heroes. The i-th hero can defeat monsters attacking the i-th or (i+1)-th town, for a total of at most B_i monsters.

What is the maximum total number of monsters the heroes can cooperate to defeat?

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^9

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_{N+1}
B_1 B_2 ... B_N

Output

Print the maximum total number of monsters the heroes can defeat.

Sample Input 1

2
3 5 2
4 5

Sample Output 1

9

If the heroes choose the monsters to defeat as follows, they can defeat nine monsters in total, which is the maximum result.

The first hero defeats two monsters attacking the first town and two monsters attacking the second town.

The second hero defeats three monsters attacking the second town and two monsters attacking the third town.

Sample Input 2

3
5 6 3 8
5 100 8

Sample Output 2

22

Sample Input 3

2
100 1 1
1 100

Sample Output 3

3

Solution in Ruby (2.3.3) (judge verdict: Accepted)

n = gets.to_i
a_ary = gets.chomp.split(' ').map(&:to_i)
taoseru_ary = gets.chomp.split(' ').map(&:to_i)

ans = 0
taoseru_ary.each_with_index do |taoseru, idx|
  # 倒せる数よりAiにいるモンスターの数が多いとき
  if a_ary[idx] >= taoseru
    ans += taoseru
  else # まだ倒せるモンスターががいる時
    # 隣町のモンスターを含めると、倒しきれないとき
    if a_ary[idx+1] + a_ary[idx] >= taoseru
      ans += taoseru
      a_ary[idx+1] = a_ary[idx+1] - (taoseru - a_ary[idx])
    else
      # 隣町のモンスターを倒しきれるとき
      ans += a_ary[idx+1] + a_ary[idx]
      a_ary[idx+1] = 0
    end
  end
end
puts ans