Lucky Numbers

using System;
  class Lucky
{
  static void Main()
  {
    //Telerik Academy
    //Two lucky numbers of 4s and 7s are entered on the console. Find the minimum operations needed to make them equal. The 2 possible operations are pair exchange or setting 4 to 7 and vice versa.
    string a = Console.ReadLine(),
           b = Console.ReadLine();

    a = a.Replace('4', '0');
    a = a.Replace('7', '1');
    b = b.Replace('4', '0');
    b = b.Replace('7', '1');

    int numsLenght = a.Length;
    int count0s = 0,
        count1s = 0;


    for (int bitposition = 0; bitposition < numsLenght; bitposition++)
    {
      if (a[bitposition] != b[bitposition])
      {
        if (a[bitposition] == '0')
        {
          count0s++;
        }
        else
        {
          count1s++;
        }
      }
    }

    int result = Math.Max(count0s, count1s);
    Console.WriteLine(result);
  }
}