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 Go (1.14.1) (judge verdict: Accepted)

package main

import (
	"fmt"
)

func main() {
	var n int
	var s string
	fmt.Scan(&n, &s)
	var r, g, b int
	for i := 0; i < n; i++ {
		if s[i] == 'R' {
			r++
		} else if s[i] == 'G' {
			g++
		} else {
			b++
		}
	}

	ans := r * g * b
	for i := 0; i < n; i++ {
		for j := i+1; j < n; j++ {
			k := 2*j - i
			if k == j || k >= n {
				continue
			}
			if s[i] != s[j] && s[j] != s[k] && s[k] != s[i] {
				ans--
			}
		}
	}
	fmt.Println(ans)
}