Letters occurence Frequency

This code was created for counting the frequency of letters occurence in a written word, phrase or expression :

star=input("enter the word , phrase sentence: ") 
str=input("enter the letter you want to know its occurence :")
print("the letter", str, "appears", star.count(str), "times")
start=input("Do you want to enter new word, phrase or senstences again ?:")
while start== "yes":
  star=input("enter the word , phrase sentence: ")  
  str=input("enter the letter you want to know its occurence :")
  print("the letter", str, "appears", star.count(str), "times") 
  if start != "yes":
    print ("Thank you!")