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 C# (.NET Core 3.1.201) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;

namespace AtCoderCs
{
    class Program
    {
        static void Main()
        {

            var Num = int.Parse(Console.ReadLine());
            var Result = 0;
            var RGBText = Console.ReadLine().ToArray();

            for (int i = 0; i < Num - 2; i++)
            {

                for (int j = i + 1; j < Num - 1; j++)
                {

                    if (RGBText[i] != RGBText[j])
                    {
                        Result += RGBText[(j + 1)..^0].Count(x => x != RGBText[i] && x != RGBText[j]);
                        if (j + (j - i) < Num)
                        {

                            var Condition = RGBText[j + j - i];
                            if (Condition != RGBText[i] && Condition != RGBText[j]) Result--;

                        }
                    }
                }
            }

            Console.WriteLine(Result);
        }


    }
}