Caesar cipher decryption problem

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C#

Im trying to do an exercise in a programming book, one of the exercises being decrypting ciphertext that uses the Caesar cipher with an unknown shift.

Here is the ciphertext:

HVWGWGHVSPSGHQCADIHSFSLSFQWGSWVOJSSJSFSLDSFWSBQSR

I don't know the expected output because I haven't determined the algorithm yet.

I've written the following algorithm to try to decrypt it using the Caesar cipher, but this is as far as I've gotten. How do I decrypt this in C#?

What I have tried:

string s = "HVWGWGHVSPSGHQCADIHSFSLSFQWGSWVOJSSJSFSLDSFWSBQSR";

int[] freq = new int[26];

for (int i = 0; i < s.Length; i++)
{
  // converting 
  string temp = s.Substring(i, 1); 

  // converting to an array
  int itemp = (int)temp.ToCharArray()[0];

  freq[itemp - 65]++;  
}

for (int i = 0; i < 26; i++)
{
  Console.WriteLine(i + " " + freq[i]);
}