import java.math.BigDecimal;
import java.math.MathContext;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
  int vl, vr;
  int d;
  int w;

  /**
   * 想定解で誤差が酷いものになってないかチェック
   */
  BigDecimal checkai() {
    BigDecimal vl = new BigDecimal(this.vl, MathContext.DECIMAL128);
    BigDecimal vr = new BigDecimal(this.vr, MathContext.DECIMAL128);
    BigDecimal d = new BigDecimal(this.d, MathContext.DECIMAL128);
    BigDecimal w = new BigDecimal(this.w, MathContext.DECIMAL128);

    return w.multiply(d).divide(vl.add(vr), MathContext.DECIMAL128);
  }

  /**
   * 想定解
   * 算数
   */
  double souteikai() {
    double vl = (double) this.vl;
    double vr = (double) this.vr;
    double d = (double) this.d;
    double w = (double) this.w;

    return w * d / (vl + vr);
  }

  /**
   * 誤解
   * 数学
   * 誤差
   */
  double mendoksai() {
    double vl = (double) this.vl;
    double vr = (double) this.vr;
    double d = (double) this.d;
    double w = (double) this.w;

    double t1 = d / (w + vr);
    double d1 = d - t1 * (vl + vr);
    double h1 = d1 / d;
    double t2 = d1 / (w + vl);
    double d2 = d1 - t2 * (vl + vr);
    double h2 = d2 / d1;

    return w * (t1 + t2) / (1 - h1 * h2);
  }

  /**
   * 誤解
   * プログラムの力任せ
   * TLE、誤差
   */
  double gokai() {
    double vl = (double) this.vl;
    double vr = (double) this.vr;
    double d = (double) this.d;
    double w = (double) this.w;

    double ans = 0;

    while (d > 1e-7) {
      double t = d / (w + vr);
      ans += w * t;
      d -= t * (vl + vr);
      double tmp = vr;
      vr = vl;
      vl = tmp;
    }

    return ans;
  }

  void run() {
    vl = sc.nextInt();
    vr = sc.nextInt();
    d = sc.nextInt();
    w = sc.nextInt();

    assert (1 <= vl && vl <= 1_000_000_000L);
    assert (1 <= vr && vr <= 1_000_000_000L);
    assert (1 <= d && d <= 1_000_000_000L);
    assert (1 <= w && w <= 1_000_000_000L);
    assert (Math.max(vl, vr) < w);

    BigDecimal s = checkai();
    assert (Math.abs(BigDecimal.valueOf(souteikai()).subtract(s).doubleValue()) < 1e-6);
    if (max(d, w) / max(vl, vr) < 100_000) {
      assert (Math.abs(BigDecimal.valueOf(mendoksai()).subtract(s).doubleValue()) < 1e-6);
    }
    if (max(vl, vr, d, w) <= 100_000) {
      assert (Math.abs(BigDecimal.valueOf(gokai()).subtract(s).doubleValue()) < 1e-6);
    }

    System.out.println(s.toPlainString());
  }

  Scanner sc = new Scanner(System.in);

  public static void main(String[] args) {
    new Main().run();
  }

  long max(long... a) {
    long r = Long.MIN_VALUE;
    for (long v : a) {
      r = Math.max(r, v);
    }
    return r;
  }
}