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 1.8.0) (judge verdict: Accepted)

import java.util.*;
public class Main {
	public static void main(String[] args){
		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<n; i++ ) {
			if ( s.charAt(i)=='R' ) {
				r++;
			} else if ( s.charAt(i)=='G' ) {
				g++;
			} else {
				b++;
			}
		}

		if ( r*g*b==0 ) {
			System.out.println(0);
			System.exit(0);
		}

		long f = 0;
		for ( int i=0; i<n-2; i++ ) {
			for ( int j=i+1; j<n; j++ ) {
				int k = 2*j-i;
				if ( k < n ) {
					if ( s.charAt(i)!=s.charAt(j) && s.charAt(j)!=s.charAt(k) && s.charAt(k)!=s.charAt(i) ) {
						f++;
					}
				}
			}
		}

		System.out.println(r*g*b-f);
	}

}