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 PyPy3 (7.3.0) (judge verdict: Accepted)

n = int(input())
s = str(input())
count = 0
sum_RGB = s.count("R") * s.count("G") * s.count("B")
for i in range(n):
    for j in range(i, n):  # rangeをiからにすることでループ減らせる
        # for k in range(n): # sum_RGBから減らしていくのでｋがi,jで決まる
        k = 2*j - i
        if k < n:
            a = (s[i], s[j], s[k])
            # print(a)
            a_set = set(a)
            # print(a_set)
            if len(a_set) == 3:
                count += 1

print(sum_RGB - count)