#include <bits/stdc++.h>
#define REP(i,n) for(int i = 0; i < (int)(n); ++i)
using namespace std;

int main(){
    int N, M, X, Y;
    cin >> N >> M;
    cin >> X >> Y;
    int x = min(X,Y);
    int y = max(X,Y);
    bool flag = false;
    int m = M;
    while(2*m<=N){
        if(y==2*m+x or y==2*m-x+1){flag = true; break;}
        else m++;
    }
    cout << (flag?"YES":"NO") << endl;
    return 0;
}