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