Объяснить причину возникновения ошибки - C#

using System;
 
namespace inf
{
    class el
    {
        public string name;
        public int d;
        public int m;
        public int y;
    }
    class MainClass
    {
        public static void Main (string[] args)
        {
            int k=0;
            int md, mm, my;
            string answ="";
            int n = int.Parse (Console.ReadLine ());
            el[] mas = new el[n];
            md = 31;mm = 12;my = 2100;
            for (int i = 0; i < n; i++) 
            {
                String[] s = Console.ReadLine ().Split (' ','.');
                mas[i].name =s[0]+" "+s[1];
                mas[i].d = int.Parse (s [2]);
                mas[i].m = int.Parse (s [3]);
                mas[i].y = int.Parse (s [4]);
                if (mas [i].y < my) {
                    md = mas [i].d;
                    mm = mas [i].m;
                    my = mas [i].y;
                } else {
                    if ((mas [i].y == my) && (mas [i].m < mm))
                        mm = mas [i].m;
                    if ((mas [i].y == my) && (mas[i].n==mm) && (mas [i].d < md))
                        md = mas [i].d;
                    md = mas [i].d;
                    mm = mas [i].m;
                    my = mas [i].y;
                }
                
            }
 
            for (int i = 0; i < n; i++) {
                if ((mas [i].y == md) && (mas [i].m == mm) && (mas [i].y == my))
                    k++;
                if (k == 1)
                    answ = mas [i].name + " " + mas [i].d + "." + mas [i].m + "." + mas [i].y;
            }
 
            if (k == 1)
                Console.WriteLine (answ);
            else
                Console.WriteLine (k);
 
     }
    }
}