X, Y, Z, W = gets.split.map(&:to_i)

ok = 10 ** 6
ng = Y
loop_cnt = 10000

loop_cnt.times do
  ad = (ok + ng) / 2.0
  bq = Math.sqrt(ad ** 2 + Z ** 2)
  bp = bq + W
  ab = Math.sqrt(bp ** 2 - Y ** 2)
  area = ab * ad 

  if area >= X
    ok = ad 
  else
    ng = ad 
  end
end

ad = ok
ab = Rational(X, ad)
area = ad * ab - Rational(Y * ab, 2) - Rational(ad * Z, 2) - Rational((ad - Y) * (ab - Z), 2)

puts "%.12f" % area