C# Online Compiler | .NET Fiddle

using System;
					
public class Program
{
	public static void Main()
	{
		float n, lado, ap, area;
		Console.WriteLine("¿Cuántos lados tiene el polígono?");		
		n = Convert.ToInt32(Console.ReadLine());
		Console.WriteLine("¿Cuántos miden esos lados?");
		lado = Convert.ToInt32(Console.ReadLine());
		Console.WriteLine("¿Cuánto mide el apotema?");
		ap = Convert.ToInt32(Console.ReadLine());
		area = calcular_area(n, lado, ap);
		Console.WriteLine("El área del polígono es de " + area.ToString());
	}
	
	public static float calcular_area(float n, float lado, float ap)
	{
		float area;
		area = perimetro(n, lado) * ap / 2;
		return area;
	}
	
	public static float perimetro(float n, float lado)
	{
		return n * lado;
	}
}

 
using System;
                    
public class Program
{
    public static void Main()
    {
        float n, lado, ap, area;
        Console.WriteLine("¿Cuántos lados tiene el polígono?");     
        n = Convert.ToInt32(Console.ReadLine());
        Console.WriteLine("¿Cuántos miden esos lados?");
        lado = Convert.ToInt32(Console.ReadLine());
        Console.WriteLine("¿Cuánto mide el apotema?");
        ap = Convert.ToInt32(Console.ReadLine());
        area = calcular_area(n, lado, ap);
        Console.WriteLine("El área del polígono es de " + area.ToString());
    }
    
    public static float calcular_area(float n, float lado, float ap)
    {
        float area;
        area = perimetro(n, lado) * ap / 2;
        return area;
    }
    
    public static float perimetro(float n, float lado)
    {
        return n * lado;
    }
}

Compilation error (line 2, col 7): The type or namespace name 'BenchmarkDotNet' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 3, col 7): The type or namespace name 'BenchmarkDotNet' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 9, col 6): The type or namespace name 'BenchmarkAttribute' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 9, col 6): The type or namespace name 'Benchmark' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 15, col 6): The type or namespace name 'BenchmarkAttribute' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 15, col 6): The type or namespace name 'Benchmark' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 34, col 9): The name 'BenchmarkRunner' does not exist in the current context