output_description: Print the number of triplets in question.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      S
attempt: N = int(input())
  S = input()
  ret = 0
  for j in range(1, N - 1):
      if S[j] == "R":
          ret += S[:j].count("G") * S[j + 1 :].count("B")
          ret += S[:j].count("B") * S[j + 1 :].count("G")
          for i in range(j):
              if j + j - i < N:
                  if S[i] != S[j + (j - i)] and S[i] != "R" and S[j + (j - i)] != "R":
                      ret -= 1
      elif S[j] == "G":
          ret += S[:j].count("B") * S[j + 1 :].count("R")
          ret += S[:j].count("R") * S[j + 1 :].count("B")
          for i in range(j):
              if j + j - i < N:
                  if S[i] != S[j + (j - i)] and S[i] != "G" and S[j + (j - i)] != "G":
                      ret -= 1
      elif S[j] == "B":
          ret += S[:j].count("R") * S[j + 1 :].count("G")
          ret += S[:j].count("G") * S[j + 1 :].count("R")
          for i in range(j):
              if j + j - i < N:
                  if S[i] != S[j + (j - i)] and S[i] != "B" and S[j + (j - i)] != "B":
                      ret -= 1
  print(ret)
  
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"}]