main :: IO ()
main = do
  n <- (read :: String -> Integer) <$> getLine
  print $ ans n

ans :: Integer -> Integer
ans n
  | odd n = 0
  | even n = let m = n `div` 10 in m + blind m where
  blind k
   | k < 5 = 0
   | k > 4 = let l = k `div` 5 in l + blind l