// No.396 クラス替え
// https://yukicoder.me/problems/no/396
//
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
using namespace std;

string solve(int M, int X, int Y);
int check_class(int a, int b);

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    int N, M, X, Y;
    cin >> N >> M >> X >> Y;
    string ans = solve(M, X, Y);
    cout << ans << endl;
}

string solve(int M, int X, int Y) {
    int hanako = check_class(X, M);
    int taro = check_class(Y, M);
    if (hanako == taro)
        return "YES";
    else
        return "NO";
}

int check_class(int a, int b) {
    int res = -1;
    int q = (int)a / b;
    int r = a % b;
    if (q % 2 == 0) {
        if (r == 0 || r == 1)
            res = 1;
        else
            res = r;
    } else {
        if (r == 0 || r == 1) {
            res = b;
        } else
            res = b - r + 1;
    }
    return res;
}