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(); }