#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define SZ(a) (int) a.size()
#define endl '\n'
#define X first
#define Y second

void solve(){
  double d, s, r, o;
  cin >> d >> s >> r >> o;
  const double pi = 3.14159265;
  double ang = sin(pi*d/2/s);
  if((s-r)*ang< r - o/2){
    cout << "-1.0\n";
    return;
  } 
  if(r*ang > r - o/2){
    cout << "-2.0\n";
    return;
  }
  cout << setprecision(2) << (ang*(s-r)+o/2.0-r)/ang << "\n";
}

int main() {
  ios::sync_with_stdio(0); cin.tie(0);
  int t = 1;
  // cin >> t;
  while (t--) {
      solve();
  }
  return 0;
}