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)

fun main(args: Array<String>) {
    var n = readLine()!!.toInt()
    var s = readLine()!!.toString()
    var c = mutableMapOf<Char, Int>('R' to 0, 'G' to 0, 'B' to 0)
    for (i in s) {
        c[i] = c[i]!! + 1
    }
    var diff = 0
    var ans = 1L
    for (d in 1..((n-1)/2)){
        for (i in 0..(n-d-d-1)) {
            if (s[i] != s[i+d] && s[i+d] != s[i+d+d] && s[i+d+d] != s[i]) {
                diff += 1
            }
        }
    }
    for ((x, y) in c){ans *= y}
    println(ans-diff)
}