#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) for(int i = 0; i < (n); ++i)
using namespace std;
typedef long long ll;
const int INF = 1e9;
const double PI = acos(-1.0);

int func(int N, int M, int X) {
	int x = X % M;
	int y = X / M;
	if(x == 0) {
		if(y % 2) return M;
		else return 1;
	} else {
		if(y % 2) return M - x + 1;
		else return x;
	}
}

int main(void) {
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int N, M;
	int X, Y;
	cin >> N >> M;
	cin >> X >> Y;
	
	if(func(N, M, X) == func(N, M, Y)) cout << "YES";
	else cout << "NO";
		
	return 0;
}