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# (Mono-csc 3.5.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = int.Parse(Console.ReadLine());
            var s = Console.ReadLine();

            var r_num = s.Length - s.Replace("R", "").Length;
            var g_num = s.Length - s.Replace("G", "").Length;
            var b_num = s.Length - s.Replace("B", "").Length;

            var sum = r_num * g_num * b_num;

            for(var i = 2; i < n; i++)
            {
                var a = s[i];

                for (var j = (i+1)>>1; j < i; j++)
                {
                    var b = s[j];
                    var c = s[j - (i - j)];

                    if (a != b && b != c && c != a)
                    {
                        sum--;
                    }
                }
            }

            Console.WriteLine(sum);
        }
    }
}