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.split(' ').map(&:to_i)
b_ary = gets.split(' ').map(&:to_i)

kill_count = 0

a_ary.each_with_index.reverse_each do |monster_count, idx|
  if b_ary[idx] != nil && b_ary[idx] > 0
    if b_ary[idx] >= monster_count
      b_ary[idx] -= monster_count
      kill_count += monster_count
      next
    else
      kill_count += b_ary[idx]
      monster_count -= b_ary[idx]
    end
  end

  if idx != 0 && b_ary[idx - 1] != nil && b_ary[idx - 1] > 0
    if b_ary[idx - 1] >= monster_count
      b_ary[idx - 1] -= monster_count
      kill_count += monster_count
    else
      kill_count += b_ary[idx - 1]
      b_ary[idx - 1] = 0
    end
  end
end

puts kill_count