def calc()
    #input
    input = gets.split(' ').map {|s| s.to_i}
    raise "Wrong number of arguments" if input.length != 4
    r, t, vmouse, vcat, = input
    #calculate
    perim = 2 * Math::PI * r
    dist = [(t * vmouse), perim/2].min
    vcat-vmouse <= 0 ? result = -1 : result = dist / (vcat-vmouse)
    #output
    puts sprintf('%.9f', result)
end