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)

function main()
  n = parse(Int, readline())
  s = chomp(readline())

  # # 3文字未満の場合、条件を満たす組は存在しない
  if n < 3
    println(0)
    exit()
  end

  # 間隔が等しい場合の数を先に求める
  cnt = 0
  for d in 1:div(n-1,2)
    for i in 1:(n-2*d)
      j = i + d
      k = j + d
      if s[i] != s[j] && s[j] != s[k] && s[k] != s[i]
        cnt += 1
      end
    end
  end

  r = 0
  g = 0
  b = 0
  ans = 0
  for i in 1:n
    if s[i] == 'R'
      ans += g * b
      r += 1
    elseif s[i] == 'G'
      ans += r * b
      g += 1
    elseif s[i] == 'B'
      ans += r * g
      b += 1
    end
  end

  println(ans - cnt)
end

main()