Why won't my function return the letters inside the string?

def substring_between_letters(word, start, end):
  strt = word.find(start)
  endd = word.find(end)
  if strt or endd == -1:
    return word
  else:
    return word[strt+1:endd]

print(substring_between_letters("apple", "p", "e"))
print(substring_between_letters("apple", "p", "c"))

In both scenarios I am returning apple.

strt finds the index position of start endd finds the index position of end and if either of these are -1 then return the word else return what is between the letters