output_description: Print the number of triplets in question.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      S
attempt: m = int(input())
  N = input()
  cnt = N.count("R") * N.count("G") * N.count("B")
  # print(cnt)
  for i in range(m - 2):
      # print(i,'*',int((m+i)/2))
      for j in range(i + 1, int((m + i - 1) / 2) + 1):
          k = 2 * j - i
          # print(i,j,k)
          if N[k] != N[j] and N[j] != N[i] and N[i] != N[k]:
              # print(i,j,k)
              cnt -= 1
  print(cnt)
  
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"}]