---

using System;

namespace Example
{
    class Program
    {
        static void Main()
        {
            Console.Write("a = ");
            double a = double.Parse(Console.ReadLine());

            Console.Write("b = ");
            double b = double.Parse(Console.ReadLine());

            Console.Write("c = ");
            double c = double.Parse(Console.ReadLine());

            Console.Write("d = ");
            double d = double.Parse(Console.ReadLine());

            double l = Math.Sqrt((c - a) * (c - a) +
                                 (d - b) * (d - b));

            Console.WriteLine("Расстояние = " + l);
        }
    }
}