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;

class D
{
	static void Main()
	{
		var n = int.Parse(Console.ReadLine());
		var s = Console.ReadLine();
		int r = 0, g = 0, b = 0;
		for (int i = 0; i < n; i++)
		{
			if (s[i] == 'R') r++;
			else if (s[i] == 'G') g++;
			else b++;
		}
		var a = r * g * b;
		if (a == 0)
		{
			Console.WriteLine(0);
			return;
		}
		for (int i = 0; i <= n; i++)
		{
			for (int j = i + 1; j < n; j++)
			{
				if (j + j - i > n - 1) continue;
				if (s[i] != s[j] && s[i] != s[j + j - i] && s[j] != s[j + j - i]) a--;
			}
		}
		Console.WriteLine(a);
	}
}