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 Julia (1.4.0) (judge verdict: Accepted)

let
  N = parse(Int, readline())
  S = readline()
  a = 0
  r = 0
  g = 0
  b = 0
  for i = 1:N
    S[i] == 'R' && (r += 1)
    S[i] == 'G' && (g += 1)
    S[i] == 'B' && (b += 1)
  end
  a = r*g*b
  for l = 1:Int(ceil(N/2)-1), i = 1:N
    if i+2l <= N && S[i] != S[i+l] && S[i] != S[i+2l] && S[i+l] != S[i+2l]
      a -= 1
    end
  end
  println(a)
end