toBitarray :: String -> Int -> Int -> [Bool]
toBitarray s k fullN = replicate (k - 1) True ++ [c == '1' | i <- [k - 1 .. fullN - 1], let c = s !! (i - k + 1)]

min_find :: String -> Int -> Int -> Int
min_find s n len = changes
  where
    weightMin = (len + 1) `div` 2
    fullLen = n + weightMin - 1
    bits = toBitarray s weightMin fullLen
    initialHWeight = length . filter id $ take len bits
    (_, _, changes) = foldl (update weightMin len) (bits, initialHWeight, if initialHWeight < weightMin then 1 else 0) [1 .. fullLen - len]
    update wm l (xs, hw, ch) i =
      let hw' = hw - fromEnum (xs !! (i - 1)) + fromEnum (xs !! (i + l - 1))
      in if hw' < wm
           then (take (i + l - 1) xs ++ [True] ++ drop (i + l) xs, hw' + 1, ch + 1)
           else (xs, hw', ch)

main :: IO ()
main = do
  n <- readLn
  s <- getLine
  print $ min_find s n 3