C# Console Application When variable change connection automatically change

Console.WriteLine("Set firstValue");
decimal first = Convert.ToDecimal(Console.ReadLine());

first variable

Console.WriteLine("Set Changeable Value");
decimal changeable = first + 5;

second value depends on first value

Console.WriteLine("Set firsValue again");
first=Convert.ToDecimal(Console.ReadLine());

when change first value change otomatically changeable value. But I want to use observer and strategy design pattern really I mess up