local N = io.read("*n")
io.read("*l")
local S = io.read("*l")

local S_bytes = {S:byte(1, N)}
local R_count, G_count, B_count = 0LL, 0LL, 0LL
local R, G, B = string.byte("RGB", 1, 3)

for i = 1, N do
    if S_bytes[i] == R then
        R_count = R_count + 1
    elseif S_bytes[i] == G then
        G_count = G_count + 1
    else
        B_count = B_count + 1
    end
end

local minus_count = 0LL
for i = 1, math.modf(N/2) do
    for j = 1, N - i * 2 do
        print(i, j)
        if S_bytes[j] ~= S_bytes[j + i]
        and S_bytes[j + i] ~= S_bytes[j + 2 * i]
        and S_bytes[j] ~= S_bytes[j + 2 * i] then
            minus_count = minus_count + 1
        end
    end
end

print(tostring(R_count * G_count * B_count - minus_count):sub(1, -3))