fn main() {
    input!{
		mut x: i128, n: i128,
	}

	let mut f = false;
	if x < 0 {
		x *= -1;
		f = true;
	}

	// n項目が正のまま
	let acc_val = calc_acc_val(n);
	if x - acc_val >= 0 {
		println!("{}", ans_val(x, acc_val, f));
		return;
	}

	// 正、負が変わる場合は、その地点を見つける
	let mut left = -1i128;
	let mut right = 1i128 << 60;
	for _ in 0..70 {
		let mid = (left + right) / 2;
		if x - calc_acc_val(mid) > 0 {
			left = mid;
		}
		else {
			right = mid;
		}
	}

	// 初めて0以下になった地点からの項数
	let rest_cnt = n - right;
	if rest_cnt % 2 == 0 {
		let add_cnt = rest_cnt / 2;
		println!("{}", ans_val(x - add_cnt, calc_acc_val(right), f))
	}
	else {
		let add_cnt = (rest_cnt+1) / 2;
		println!("{}", ans_val(x + add_cnt, calc_acc_val(left), f))
	}


}

fn calc_acc_val(x: i128) -> i128 {
	x * (1+x) / 2
}

fn ans_val(x: i128, acc_x: i128, f: bool) -> i128 {
	let mut ret = x - acc_x;
	if f { ret *= -1; }
	ret
}

// const MOD17: usize = 1000000007;
// const MOD93: usize = 998244353;
// const INF: usize = 1 << 60;
// let dx = vec![!0, 0, 1, 0]; // 上左下右
// let dy = vec![0, !0, 0, 1]; // 上左下右
// let d = vec!{(!0, 0), (0, !0), (1, 0), (0, 1)}; // 上左下右

#[allow(unused)]
use proconio::{input, marker::Chars, marker::Usize1};

#[allow(unused)]
use std::{
	mem::swap,
	cmp::min, cmp::max,
	cmp::Reverse,
	collections::HashSet, collections::BTreeSet,
	collections::HashMap, collections::BTreeMap,
	collections::BinaryHeap,
	collections::VecDeque,
	iter::FromIterator,
};