program ATCoder
implicit none
character*10::s
integer::i,l,count=0,ans=0
read*,s
s=trim(adjustl(s))
l=len_trim(s)
!print*,l
do i=1,l
   if(s(i:i)=='A'.or.s(i:i)=='T'.or.s(i:i)=='C'.or.s(i:i)=='G')then
      count=count+1
   else
      if(count>=ans)then
         ans=count
         count=0
      else
         count=0
      end if
   end if
end do

print*,ans
!print*,s

end program ATCoder
