use std::env::args;
use std::fs::File;
use std::io::BufRead;
use std::io::BufReader;

fn split_once<'a>(s: &'a str, sep: &str) -> (&'a str, &'a str) {
    let i = s.find(sep).unwrap();
    let left = &s[..i];
    let right = &s[i+sep.len()..];
    return (left, right);
}

fn get_data() -> (i16, i16, i16, i16) {
    let path = args().nth(1).unwrap();
    let file = File::open(path).unwrap();

    let line = BufReader::new(file).lines().next().unwrap().unwrap();
    let (_, rest) = split_once(&line, "target area: ");
    let (x1, y1) = split_once(&rest, ", ");
    let (_, x2) = split_once(&x1, "=");
    let (_, y2) = split_once(&y1, "=");
    let (x21, x22) = split_once(&x2, "..");
    let (y21, y22) = split_once(&y2, "..");

    let x_min = x21.parse::<i16>().unwrap();
    let x_max = x22.parse::<i16>().unwrap();
    let y_min = y21.parse::<i16>().unwrap();
    let y_max = y22.parse::<i16>().unwrap();

    // inclusive
    return (x_min, x_max, y_min, y_max);
}

fn main() {
    let (x_min, x_max, y_min, y_max) = get_data();

    assert!(x_min > 0);
    assert!(x_max > 0);
    assert!(y_min < 0);
    assert!(y_max < 0);

    let mut max_y = 0;

    // The probe reaches its heighest point after v steps
    // after v further steps it is back to 0.
    // The next step will take it to -(v + 1).
    // That means there is no chance of getting to the
    // target area if -(v + 1) < y_min.

    'vy: for vy in (y_min..-y_min).rev() {
        for vx in 1..=x_max+1 {
            let mut x = 0;
            let mut y = 0;
            let mut mvx = vx;
            let mut mvy = vy;
            while x <= x_max && y >= y_min {
                x += mvx;
                y += mvy;
                if mvx > 0 {
                    mvx -= 1;
                }
                mvy -= 1;
                if x >= x_min && x <= x_max && y >= y_min && y <= y_max {
                    max_y = vy * (vy + 1) / 2;
                    break 'vy;
                }
            }
        }
    }

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