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;
using System.Collections.Generic;

namespace 練習問題
{
    class Program {
        static void Main(string[] args) {
            var sw = new System.Diagnostics.Stopwatch();
            sw.Start();
            List<int> pattern = new List<int>();
            int c = 0;
            string s = ""; 
            int result = 0;
            // Si != Sj && Si != Sk && Sj != Sk
            // j - i != k - j
            debug Debug = new debug();
            if (args.Length == 0){
                Debug.FlagFalse();
                /*
                string[] input = Console.ReadLine().Split(' ');
                int a = int.Parse(input[0]);
                int b = int.Parse(input[1]);
                */
                c = int.Parse(Console.ReadLine());
                s = Console.ReadLine();
                // string d = Console.ReadLine();

                //string[] input = Console.ReadLine().Split(' ');
            } else {
                Debug.FlagTrue();
                if (args[0] == "1"){
                    pattern.Add(4);
                    pattern.Add(3);
                    c = pattern[0];
                    s = "RRGB";
                } else if (args[0] == "2"){
                    pattern.Add(39);
                    pattern.Add(4);
                    c = pattern[0];
                    s = "RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB";
                } else {
                    c = 0;
                    s = "RGB";
                }
            }
            for (int i = 0; i < s.Length; i++) {
                for (int j = i; j < s.Length; j++) {
                    for (int k = j; k < s.Length; k++) {
                        if (s.Substring(i,1) != s.Substring(j,1) 
                        && s.Substring(i,1) != s.Substring(k,1) 
                        && s.Substring(j,1) != s.Substring(k,1)){
                            if (j - i != k - j){
                                result++;
                            }
                        }
                    }
                }
            }
            Console.WriteLine(result);
            sw.Stop();
            Console.WriteLine("■処理Aにかかった時間");
            TimeSpan ts = sw.Elapsed;
            Console.WriteLine($"　{ts}");
        }
        
        static int intPow(int target,int kaisu){
            int startTarget = target;
            for (int i = 1; i < kaisu; i++)
            {
                target *= startTarget;
            }
            return target;
        }

        static void bubble_sort(List<List<int>> a) {
            int count=0;
            for (int i = 0; i < a.Count; i++) {
                for (int j = 0; j < a.Count-i-1; j++) {
                    // if (a[j] == a[a.Count - 1]){
                    //     continue;
                    // }
                    count++;
                    if (a[j][1] > a[j+1][1]){
                        int localsave = 0;
                        localsave = a[j+1][1];
                        a[j+1][1] = a[j][1];
                        a[j][1] = localsave;
                        localsave = a[j][0];
                        a[j][0] = a[j+1][0];
                        a[j+1][0] = localsave;
                    }
                }
                //debug(a,"途中" + i);
            }
            // debug(a,"result");
            //Console.WriteLine("回数"+count);
            for (int i = 0; i < a.Count; i++)
            {
                Console.WriteLine(a[i][0]+" "+a[i][1]);
            }
        }
        /* 標準入力と標準出力が使えるかのチェック
        static void echo() {
            string echotest = Console.ReadLine();
            Console.WriteLine(echotest);
        }

        static void HelloWorld(){
            Console.WriteLine("Hello World!");
        }
        */
    }

    public class debug{ //宣言時は、「 debug Debug = new debug(); 」推奨
        bool debug_flag = true;
        public void Log(string str){
            if (debug_flag == true){
                Console.WriteLine(str);
            }
        }
        public void Log(int str){
            if (debug_flag == true){
                Console.WriteLine(str);
            }
        }
        public void Log(float str){
            if (debug_flag == true){
                Console.WriteLine(str);
            }
        }
        public void Log(object obj){
            if (debug_flag == true){
                Console.WriteLine(obj);
            }
        }

        public void Log(int[] arr){
            if (debug_flag == true){
                Console.WriteLine(string.Join(",", arr));
            }
        }
        public void Log(string[] arr){
            if (debug_flag == true){
                Console.WriteLine(string.Join(",", arr));
            }
        }
        public void Log(List<int> arr){
            if (debug_flag == true){
                string shuturyoku = "";
                for (int i = 0; i < arr.Count; i++) {
                    shuturyoku += arr[i] + ",";
                }
                Console.WriteLine(shuturyoku.TrimEnd(','));
            }
        }

        public void Log(List<string> arr){
            if (debug_flag == true){
                string shuturyoku = "";
                for (int i = 0; i < arr.Count; i++) {
                    shuturyoku += arr[i] + ",";
                }
                Console.WriteLine(shuturyoku.TrimEnd(','));
            }
        }

        public void Log(List<float> arr){
            if (debug_flag == true){
                string shuturyoku = "";
                for (int i = 0; i < arr.Count; i++) {
                    shuturyoku += arr[i] + ",";
                }
                Console.WriteLine(shuturyoku.TrimEnd(','));
            }
        }
        public void Log(){
            if (debug_flag == true){
                Console.WriteLine("アタイイレナサイ");
            }
        }
        public void FlagFalse(){
            debug_flag = false;
        }
        public void FlagTrue(){
            debug_flag = true;
        }
    }
}