use std::io::Write;
use std::collections::*;

type Map<K, V> = BTreeMap<K, V>;
type Set<T> = BTreeSet<T>;
type Deque<T> = VecDeque<T>;

fn main() {
    input! {
        h: usize,
        w: usize,
        cnt: usize,
        q: usize,
    }
    let inf = std::i32::MAX / 10;
    let mut dp = vec![vec![vec![inf; w + 1]; h + 1]; 2];
    dp[0][h][w] = 0;
    let mut deq = Deque::new();
    deq.push_back((0, h, w));
    while let Some((p, a, b)) = deq.pop_front() {
        let d = dp[p][a][b] + 1;
        if p == 0 {
            for i in 0..=a {
                for j in 0..=b {
                    if i + j > cnt {
                        continue;
                    }
                    if a - i > 0 && a - i + q < b - j {
                        continue;
                    }
                    if dp[p ^ 1][a - i][b - j].chmin(d) {
                        deq.push_back((p ^ 1, a - i, b - j));
                    }
                }
            }
        } else {
            let (x, y) = (h, w);
            for i in 0..=(x - a) {
                for j in 0..=(y - b) {
                    if i + j > cnt {
                        continue;
                    }
                    if x - a - i > 0 && x - a - i + q < y - b - j {
                        continue;
                    }
                    if dp[p ^ 1][a + i][b + j].chmin(d) {
                        deq.push_back((p ^ 1, a + i, b + j));
                    }
                }
            }
        }
    }
    let mut ans = (0..=w).map(|y| dp[1][0][y]).min().unwrap();
    if ans == inf {
        ans = -1;
    }
    println!("{}", ans);
}

// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[macro_export]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[macro_export]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[macro_export]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------
// ---------- begin chmin, chmax ----------
pub trait ChangeMinMax {
    fn chmin(&mut self, x: Self) -> bool;
    fn chmax(&mut self, x: Self) -> bool;
}

impl<T: PartialOrd> ChangeMinMax for T {
    fn chmin(&mut self, x: Self) -> bool {
        *self > x && {
            *self = x;
            true
        }
    }
    fn chmax(&mut self, x: Self) -> bool {
        *self < x && {
            *self = x;
            true
        }
    }
}
// ---------- end chmin, chmax ----------