def calc_distance(x, y) #x, y???????´???°2??????1?¬??????????
  require 'bigdecimal'
  distance = ((y[0] - x[0])**2 + (y[1] - x[1])**2) ** 0.5
  distance_approximately = BigDecimal(distance.to_s).floor(8).to_f
  return distance_approximately #P1??¨P2????????¢
end

input_values = gets.chomp.split.map(&:to_f)
x = [input_values[0], input_values[1]]
y = [input_values[2], input_values[3]]
puts calc_distance(x, y)