program main
  implicit none

  integer :: n, i, j, k, cnt, r, g, b, dis
  character(4000) :: s

  read *, n
  read *, s

  r = 0
  g = 0
  b = 0
  do i = 1, n
     select case(s(i:i))
     case('R')
        r = r + 1
     case('G')
        g = g + 1
     case('B')
        b = b + 1
     end select
  end do
  
  cnt = 0
  do dis = 1, n/2
     do i = 1, n-2*dis
        if (s(i:i) /= s(i+dis:i+dis) &
             .and. s(i:i) /= s(i+2*dis:i+2*dis) &
               .and. s(i+dis:i+dis) /= s(i+2*dis:i+2*dis)) then
           cnt = cnt + 1
        end if
     end do
  end do

  print *, r*g*b - cnt
  
  stop
end program main
