//피보나치 수열: 1 1 2 3 5 8 13 21 ...
using System;

class WhileFibonacci
{
    static void Main()
    {
        int first = 0;
        int second = 1;

        while (second <= 20)
        {
            Console.WriteLine(second);
            int temp = first + second;
            first = second;
            second = temp;
        }
    }
}