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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;

public class Main {

    public static void main(String[] args) {
        // write your code here
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        String s = sc.next();

        long r = 0;
        long g = 0;
        long b = 0;

        for(int i = 0 ; i < s.length() ; i++){
            switch (s.charAt(i)){
                case 'R':
                    r++;
                    break;
                case 'G':
                    g++;
                    break;
                case 'B':
                    b++;
                    break;
            }
        }

        long sum = r*g*b;

        for(int i = 0 ; i < n ; i++){
            for(int d = 1 ; i + 2*d < n ; d++){
                if(s.charAt(i) != s.charAt(i+d) && s.charAt(i+d) != s.charAt(i+2*d) && s.charAt(i+2*d) != s.charAt(i)){
                    sum--;
                }
            }
        }

        System.out.println(sum);
    }
}