output_description: Print the number of triplets in question.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      S
attempt: x = int(input())
  s = list(input())
  di = {"R": 0, "G": 0, "B": 0}
  for i in s:
      di[i] += 1
  c = 0  # 重複を数える
  p = (len(s) - 3) // 2
  for i in range(len(s) - 2):
      j = 0
      while j < p + 1 and i + (2 * j) <= len(s) - 1:
          ss = s[i] + s[i + j] + s[i + (2 * j)]
          j += 1
          if (
              ss == "GBR"
              or ss == "GRB"
              or ss == "RBG"
              or ss == "RGB"
              or ss == "BGR"
              or ss == "BRG"
          ):
              c += 1
  print(di["R"] * di["G"] * di["B"] - c)
  
problem_description: 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.
samples: [{"input": "4\n    RRGB", "output": "1\n    \n\nOnly the triplet (1,~3,~4) satisfies both conditions. The triplet (2,~3,~4)\nsatisfies the first condition but not the second, so it does not count.\n\n* * *"}, {"input": "39\n    RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB", "output": "1800"}]