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{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiWayIf #-}
-- {-# LANGUAGE NegativeLiterals #-}
-- {-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# OPTIONS_GHC -Wno-deferred-type-errors #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE Strict #-}
{-# LANGUAGE StrictData #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE ForeignFunctionInterface, GHCForeignImportPrim, MagicHash, UnboxedTuples, UnliftedFFITypes #-}
{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -O0 #-}
{-# OPTIONS_GHC -O2 #-}
{-# OPTIONS_GHC -Wno-deferred-type-errors #-}
{-# OPTIONS_GHC -Wno-missing-import-lists #-}
{-# OPTIONS_GHC -Wno-unused-imports #-}
{-# OPTIONS_GHC -Wno-incomplete-patterns #-}


{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE FlexibleInstances   #-}
{-# LANGUAGE UndecidableInstances#-}
{-# OPTIONS_GHC -Wno-overlapping-patterns #-}

{-# LANGUAGE MultiParamTypeClasses #-}
{-# OPTIONS_GHC -Wno-unrecognised-pragmas #-}
{-# HLINT ignore "Use camelCase" #-}

module Main where
import Data.Bool
import Control.Applicative
import Data.Ord
import Control.Monad.State
import qualified Control.Monad as CM
import qualified Control.Monad.ST  as ST
import qualified Data.Array             as A
import qualified Data.Array.IO                 as AIO
import qualified Data.Array.ST                 as AST
import qualified Data.Array.Unboxed            as AU
import qualified Data.Bits                     as Bits
import qualified Data.ByteString.Char8         as BS
import qualified Data.Char as Char
import qualified Data.Complex                  as Comp
import Data.Function(fix)

import qualified Data.IORef                    as IO
import qualified Data.List                     as L
import qualified Data.Map.Strict               as Map
import qualified Data.IntMap                   as MI
import qualified Data.Maybe                    as May
import qualified Data.STRef                    as STR
import qualified Data.Sequence                 as Seq
import qualified Data.Set                      as Set
import qualified Data.Vector                   as V
import qualified Data.Vector.Generic           as G
import qualified Data.Vector.Mutable           as VM
import qualified Data.Vector.Unboxed           as VU
import qualified Data.Vector.Unboxed.Mutable   as VUM
import Prelude hiding(print)

mod' = 1000000007

main = do
    n <- readLn @Int
    as <- getLine
    print$solve$filter
        (liftM2(||)('A' ==) 
        (liftM2 (||)('G' ==) 
        (liftM2 (||) ('C' ==) 
        ('T' ==))) . snd) 
        $zip [1..] as

solve xs
 | length xs == 0 = 0
 | otherwise = fst $ last xs

-----------------------------------------------------------

type Moji = BS.ByteString
type LV   = V.Vector
type SV   = VU.Vector


cin::Sanao a=>IO a
cin=sanao<$>BS.getLine

print :: Show a => a -> IO ()
print x=BS.putStrLn $ BS.pack $ show x

class Sanao a where
  sanao :: Moji -> a
  {-# MINIMAL sanao #-}

instance Sanao Moji  where
  sanao = id

instance Sanao Integer where
  sanao = read.BS.unpack

instance Sanao Int where
    sanao x = case BS.readInt x of
                Nothing -> error "ここはInt"
                Just (a,_) -> a

instance Sanao Double where
    sanao x = case BS.readInt x of
                Nothing -> error "ここはDouble"
                Just (a,_) -> realToFrac a

instance Sanao [Moji] where
    sanao = BS.words

instance Sanao [Int] where
    sanao = map sanao. BS.words

instance Sanao(Int,Int) where
    sanao= (\[a, b]->(a, b)) . map sanao . BS.words

instance Sanao(Int, Int, Int) where
    sanao= (\[a, b, c]->(a, b, c)) . map sanao . BS.words

instance Sanao(Int, Int, Int, Int) where
    sanao= (\[a, b, c, d]->(a, b, c, d)) . map sanao . BS.words

instance Sanao(Double,Double) where
    sanao= (\[a, b]->(a, b)) . map sanao . BS.words

instance Sanao(Double,Double, Double) where
    sanao= (\[a, b, c]->(a, b, c)) . map sanao . BS.words

instance Sanao(Double,Double, Double, Double) where
    sanao= (\[a, b, c, d]->(a, b, c, d)) . map sanao . BS.words

instance Sanao(Moji,Int) where
    sanao= (\[a, b]->(a,sanao b)) . BS.words

instance Sanao(Moji,Int, Int) where
    sanao= (\[a, b, c]->(a,sanao b, sanao c)) . BS.words


instance Sanao(Int,Moji) where
    sanao= (\[a, b]->(sanao a,b)) . BS.words

instance Sanao(Moji,Double) where
    sanao= (\[a, b]->(a,sanao b)) . BS.words

instance Sanao(Double,Moji) where
    sanao= (\[a, b]->(sanao a,b)) . BS.words