#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
#define fi first
#define se second
#define pb push_back

int main(){
  ld x,y,z,w;
  cin>>x>>y>>z>>w;
  
  ld maxw=sqrt(y*y+(x/y)*(x/y))-sqrt(y*y+z*z);
  ld minw=sqrt(y*y+z*z)-sqrt(z*z+(x/y)*(x/y));
  if(w<minw||maxw<w){
    cout<<"WARNING"<<endl;
  }
  
  cout<<fixed<<setprecision(10);
  cout<<(x-y*z)/2<<endl;
}