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.*;
import java.lang.Math;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      	int n = sc.nextInt();
      	String s = sc.next();
      	long count = 0;
      	long rc = 0;
      	long gc = 0;
      	long bc = 0;
      	int istack;
      
      	Map cMap = new HashMap<Character,Integer>() {
          {
        	put('R',1);
        	put('G',10);
        	put('B',100);
          }
        };    	
      
      	for(int i = 0;i < n;i++){
        	if(s.charAt(i) == 'R') rc++;
          	else if(s.charAt(i) == 'G') gc++;
          	else if(s.charAt(i) == 'B') bc++;
        }
 
      	for(int i = 0;i < n;i++){
          for(int j = i+1;j < n;j++){
            if(j+(j-i) < n){
              istack = (int)cMap.get(s.charAt(i))
                      +(int)cMap.get(s.charAt(j))
                      +(int)cMap.get(s.charAt(j+(j-i)));
              if(istack == 111) count++;
            }
          }
        }
      
      	System.out.println(rc*gc*bc-count);
	}
}