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

int main(void) {
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	ll N, M;
	ll X, Y;
	cin >> N >> M;
	cin >> X >> Y;
	
	vector<ll> p;
	ll m = 1;
	bool flag = true;
	REP(i, N) {
		if(m == 1) flag = true;
		if(!flag) --m;
		p.push_back(m);
		if(flag) ++m;
		if(m == M + 1) flag = false;
	}
	
	if(p[X - 1] == p[Y - 1]) cout << "YES";
	else cout << "NO";
	
	
	
	return 0;
}