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 (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

type rgb struct {
	R, G, B int
}

func nextInt() int {
	n, _ := strconv.Atoi(nextString())
	return n
}

func nextString() string {
	sc.Scan()
	return sc.Text()
}

func main() {
	sc.Split(bufio.ScanWords)
	N := nextInt()
	S := nextString()

	rc := 0
	gc := 0
	bc := 0

	for _, s := range S {
		if s == 'R' {
			rc++
		} else if s == 'G' {
			gc++
		} else {
			bc++
		}
	}

	sum := rc * gc * bc

	for i := 0; i < N; i++ {
		for j := i + 1; j < N; j++ {
			k := j + (j - i)
			if k < N {
				str := []byte{S[i], S[j], S[k]}
				if isRGB(string(str)) {
					sum--
				}
			}
		}
	}
	fmt.Println(sum)
}

func isRGB(subString string) bool {
	switch subString {
	case "RGB":
		return true
	case "RBG":
		return true
	case "GRB":
		return true
	case "GBR":
		return true
	case "BRG":
		return true
	case "BGR":
		return true
	default:
		return false
	}
}