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 Rust (1.15.1) (judge verdict: Accepted)

use std::cmp::*;

// 標準入力から一行を読み取り、指定の型に変換する
fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

// 標準入力から一行を読み取り、空白文字で分割し、各要素を指定の型に変換する関数
fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
        .map(|e| e.parse().ok().unwrap()).collect()
}

fn main() {
    let n: usize = read();
    let mut a: Vec<u64> = read_vec();
    let b: Vec<u64> = read_vec();
    let mut killed_enemy: u64 = 0;

    // 後ろから処理
    for i in (0..n).rev()
    {
        // 討伐可能数
        let mut power = b[i];

        // i + 1番目の街の敵を可能な限り倒す
        let mut possible_subjugation = min(a[i + 1], power);
        killed_enemy += possible_subjugation;
        // 倒しただけ討伐可能数を減少
        power -= possible_subjugation;
        // 街の残敵数を減らす
        a[i + 1] -= possible_subjugation;

        // 討伐可能数が0であれば次へ
        if power == 0 { continue; }

        // i番目の街の敵を可能な限り倒す
        possible_subjugation = min(a[i], power);
        killed_enemy += possible_subjugation;
        // 街の残敵数を減らす
        a[i] -= possible_subjugation;
    }

    println!("{}", killed_enemy);
}