#include<bits/stdc++.h>
#define endl '\n'
#define int long long
#define pi acos(-1.0)
using namespace std;
double d,s,r,o;


signed main()
{
    cin>>d>>s>>r>>o;
    double x1=(double)2*(r-sin(pi*d/s/2)*r);
    double x2=(double)2*(r-sin(pi*d/s/2)*(s-r));
    double x3=s-(r-o/2)/sin(pi*d/s/2)-r;
    if(x1<o)
    {
        cout<<"-1.0"<<endl;
        return 0;
    }
    if(x2>o)
    {
        cout<<"-2.0"<<endl;
        return 0;
    }
    printf("%.1lf\n",x3);
    return 0;
}