program main
  implicit none
  integer :: N
  character(len=4000) :: S
  integer,allocatable :: nS(:)
  integer(8) :: out
  integer :: i,j,k
  integer :: iR(3)
  integer,parameter :: I_R = 1, I_G = 2, I_B = 3
  integer t1, t2, t_rate, t_max, diff
  call system_clock(t1)   ! 開始時を記録  
  read(*,*) N
  read(*,*) S
  iR(:) = 0
  do i = 1,N
     if (S(i:i) == "R") then
        iR(I_R) = iR(I_R) + 1
     end if 
     if (S(i:i) == "G") then
        iR(I_G) = iR(I_G) + 1
     end if
     if (S(i:i) == "B")  then
        iR(I_B) = iR(I_B) + 1
     end if
  end do
  out = iR(1)*iR(2)*iR(3)
  do i = 1,N
     do j = i+1,N
        k = j-i+j
        if (k <= N ) then
           if (S(i:i) /= S(j:j) &
                .and. S(i:i) /= S(k:k) &
                .and. S(j:j) /= S(k:k) ) then
              out = out - 1
           end if
        end if
     end do
  end do
  write(*,'(i0)') out
  stop
end program main