local n = io.read("*n", "*l")
local s = io.read()
local t = {}
for i = 1, n do
  t[i] = s:sub(i, i)
end

local r = 0
local g = 0
local b = 0
local rg, rb, gr, gb, br, bg = 0, 0, 0, 0, 0, 0

local ret = 0
for i = 1, n do
  if t[i] == "R" then
    ret = ret + gb + bg
    gr = gr + g
    br = br + b
    r = r + 1
  elseif t[i] == "G" then
    ret = ret + rb + br
    rg = rg + r
    bg = bg + b
    g = g + 1
  else
    ret = ret + rg + gr
    rb = rb + r
    gb = gb + g
    b = b + 1
  end
end
for mid = 1, n do
  if n < 1 + mid + mid then break end
  for left = 1, n do
    local center, right = left + mid, left + mid + mid
    if n < right then break end
    if t[left] ~= t[center] and t[center] ~= t[right] and t[left] ~= t[right] then
      ret = ret - 1
    end
  end
end
print(ret)
