Fibonacci number

Given a non-negative integer, write a method that returns n-th element of Fibonacci sequence.
Expected input and output
FibonacciNumber(3) → 2 FibonacciNumber(7) → 13
using System;

namespace CSharpExercises.Exercises.Recursion
{
    class FibonacciNumberTask
    {
        static int FibonacciNumber(int num)
        {
            return num <= 1 ? num : FibonacciNumber(num - 2) + FibonacciNumber(num - 1);
        }

        static void Main(string[] args)
        {
            Console.WriteLine(FibonacciNumber(10));  // 55
            Console.WriteLine(FibonacciNumber(0));   // 0
            Console.WriteLine(FibonacciNumber(20));  // 6765
            Console.WriteLine(FibonacciNumber(13));  // 233
        }
    }
}