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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;

namespace MyProject
{
    class Program
    {
        static void Main(string[] args)
        {
#if Local
            var sw = new Stopwatch();
            sw.Start();
            Console.SetIn(new StreamReader("input.in"));
#endif

            var solver = new Solver();
            solver.Solve();
#if Local
            Console.WriteLine($"{sw.ElapsedMilliseconds}ms");
#endif
        }
    }

    class Solver
    {
        public void Solve()
        {
            var N = int.Parse(Console.ReadLine());
            var S = Console.ReadLine();

            var answer = S.Count(elem => elem == 'R') * S.Count(elem => elem == 'G') * S.Count(elem => elem == 'B');
            foreach (var i in Enumerable.Range(0, N))
            {
                foreach (var j in Enumerable.Range(i + 1, N - i - 1))
                {
                    var k = j + j - i;
                    if (k < S.Length && k > j)
                    {
                        if (S[i] != S[j] && S[i] != S[k] && S[j] != S[k])
                        {
                            answer--;
                        }
                    }
                }
            }

            Console.WriteLine(answer);
        }
    }
}