public static void Main()
{
var fibonacciNumbers = new FibonacciNumbers();
var numberWith10000Digits = "1" + new string('0', 10000);
var series = fibonacciNumbers.GenerateFibonacciSeries(BigInteger.Parse(numberWith10000Digits));
var firstTermWithThousandDigits = series.First(n => n.ToString().Length == 1000);
var index = series.IndexOf(firstTermWithThousandDigits);
// In the problem index starts from 1
Console.WriteLine($"Index of first term with 1000 digits is {index+1}");
Console.ReadKey();
}