Problem:
Score : 400 points

Problem Statement

We have a string S of length N consisting of R, G, and B.

Find the number of triples (i,~j,~k)~(1 \leq i < j < k \leq N) that satisfy both of the following conditions:

S_i \neq S_j, S_i \neq S_k, and S_j \neq S_k.

j - i \neq k - j.

Constraints

1 \leq N \leq 4000

S is a string of length N consisting of R, G, and B.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of triplets in question.

Sample Input 1

4
RRGB

Sample Output 1

1

Only the triplet (1,~3,~4) satisfies both conditions. The triplet (2,~3,~4) satisfies the first condition but not the second, so it does not count.

Sample Input 2

39
RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB

Sample Output 2

1800

Solution in Kotlin (1.3.71) (judge verdict: Accepted)

import java.io.PrintWriter

fun main() {
    val pw = PrintWriter(System.out)

    val n = readInputLine().toInt()
    val s = readInputLine().toCharArray()

    val rCnt = s.count { it == 'R' }.toLong()
    val gCnt = s.count { it == 'G' }.toLong()
    val bCnt = s.count { it == 'B' }.toLong()

    var ans = rCnt * bCnt * gCnt

    for (i in s.indices) {
        for (j in i - 1 downTo 0) {
            if (i - (i - j) * 2 < 0) {
                break
            }
            if (s[i] != s[j] && s[i] != s[i - (i - j) * 2] && s[j] != s[i - (i - j) * 2]) {
                ans--
            }
        }
    }

    pw.println(ans)

    pw.flush()
}

fun readInputLine(): String {
    return readLine()!!
}