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: Wrong Answer)

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()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
	read::<String>().split_whitespace()
		.map(|e| e.parse().ok().unwrap()).collect()
}

fn read_vec2<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
	(0..n).map(|_| read_vec()).collect()
}

pub trait LexicalPermutation {
	/// Return `true` if the slice was permuted, `false` if it is already
	/// at the last ordered permutation.
	fn next_permutation(&mut self) -> bool;
}
impl<T> LexicalPermutation for [T] where T: Ord {
	/// Original author in Rust: Thomas Backman <serenity@exscape.org>
	fn next_permutation(&mut self) -> bool {
		// These cases only have 1 permutation each, so we can't do anything.
		if self.len() < 2 { return false; }

		// Step 1: Identify the longest, rightmost weakly decreasing part of the vector
		let mut i = self.len() - 1;
		while i > 0 && self[i-1] >= self[i] {
			i -= 1;
		}

		// If that is the entire vector, this is the last-ordered permutation.
		if i == 0 {
			return false;
		}

		// Step 2: Find the rightmost element larger than the pivot (i-1)
		let mut j = self.len() - 1;
		while j >= i && self[j] <= self[i-1]  {
			j -= 1;
		}

		// Step 3: Swap that element with the pivot
		self.swap(j, i-1);

		// Step 4: Reverse the (previously) weakly decreasing part
		self[i..].reverse();

		true
	}
}

fn main() {
	let n: usize = read();
	let a: Vec<i64> = read_vec();
	let b: Vec<i64> = read_vec();

	let mut kill: i64 = 0;
	let mut brave: i64 = 0;
	for i in 0..n {
		brave += b[i];
		kill += if a[i] < brave {
			a[i]
		} else {
			brave
		};
		if (brave-a[i]) < 0 { brave = 0; }
		else { brave -= a[i]; }
	}
	if brave > 0 {
		kill += std::cmp::min(a[n], brave);
	}

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