using System;

namespace Strings
{
class Program
{
static void Main(string[] args)
{
// Задание 1
string S = "Hello, world!";
string S1 = "Hello";
string S2 = "Hi";
int index = S.IndexOf(S1);
if (index != -1)
{
S = S.Remove(index, S1.Length).Insert(index, S2);
}
Console.WriteLine(S);

// Задание 2
string sentence = "Привет мир как дела";
string[] words = sentence.Split(' ');
string result = string.Join(".", words);
Console.WriteLine(result);
// Задание 3
string filePath = @"C:\Users\User\Documents\example.txt";
int index1 = filePath.IndexOf("\\");
int index2 = filePath.IndexOf("\\", index1 + 1);
string firstDirectory = filePath.Substring(index1 + 1, index2 - index1 - 1);
if (firstDirectory == "")
{
firstDirectory = "\\";
}
Console.WriteLine(firstDirectory);
}
}

}