fun solve(input: String): Long {
    val len = input.length 
 
    val red_list: MutableList<Int> = mutableListOf()
    val green_list: MutableList<Int> = mutableListOf()
    val blue_list: MutableList<Int> = mutableListOf()

    (0 until len).forEach { 
        when(input[it]) {
            'R' -> {
                red_list.add(it)
            }
            'G' -> {
                green_list.add(it)
            }
            'B' -> {
                blue_list.add(it)
            }
            else -> {}
        }
    }
    assert(red_list.size + green_list.size + blue_list.size == len)
    val first_matched = red_list.size.toLong() * green_list.size.toLong() * blue_list.size.toLong()
    var diff = 0L
    for(i in 0 until len) {
        for(j in (i + 1) until len) {
            val k = 2 * j - i
            diff += if(k < len && input[i] != input[j] && input[j] != input[k] && input[i] != input[k])  { 1 } else {0}
        }
    }

    return first_matched - diff
} 


fun main(args: Array<String>) {
    val N = readLine()!!.toInt()
    val S = readLine()!!
    assert(S.length == N)
 
    solve(S).run(::println)
}