doubt in c#

Question

static void Main(string[] args)
{
int x = 10;
double y = 20;

Console.WriteLine("x = {0}; y = {1}", x, y);
}
// Output: x = 10; y = 20

but when i code like this
Console.WriteLine("x = {3}; y = {4}", x, y);
codeplay ground compiler reports error ,why is it so??.Where you say any place holder can be used.

Answer 1

{0} --> here the first variable after the string (in Console.WriteLine) will be inserted, which is x
Correspondingly, {1} refers to y.

{3} and {4} don't work because they refer to nothing