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 = list(input())
  # r_pos = []
  # g_pos = []
  # b_pos = []
  # for i in range(n):
  #     st = s[i]
  #     if st == "R":
  #         r_pos.append(i)
  
  #     if st == "G":
  #         g_pos.append(i)
  
  #     if st == "B":
  #         b_pos.append(i)
  
  # r_num = len(r_pos)
  # g_num = len(g_pos)
  # b_num = len(b_pos)
  # #print(r_pos, g_pos, b_pos)
  # ans = r_num * g_num * b_num
  
  # for r in r_pos:
  #     for g in g_pos:
  #         for b in b_pos:
  #             color = [r, g, b]
  #             color.sort()
  #             x = color[0]
  #             y = color[1]
  #             z = color[2]
  #             if z - y == y - x:
  #                 ans -= 1
  # print(ans)
  
  # -----------------------------------------------------
  n = int(input())
  s = list(input())
  r_pos = []
  g_pos = []
  b_pos = []
  
  for i in range(n):
      st = s[i]
      if st == "R":
          r_pos.append(i)
  
      if st == "G":
          g_pos.append(i)
  
      if st == "B":
          b_pos.append(i)
  
  r_num = len(r_pos)
  g_num = len(g_pos)
  b_num = len(b_pos)
  
  print(r_pos, g_pos, b_pos, r_num, g_num, b_num)
  
  
  ans = r_num * g_num * b_num
  for r in r_pos:
      for g in g_pos:
          x, y = min(r, g), max(r, g)
  
          dif = y - x
          left_pos = x - dif
          right_pos = y + dif
          mid_pos = 4000
          if dif % 2 == 0:
              mid_pos = x + dif // 2
  
          left = 0
          right = 0
          mid = 0
          if left_pos in b_pos:
              left = 1
          if right_pos in b_pos:
              right = 1
          if mid_pos in b_pos:
              mid = 1
  
          ans -= left + right + mid
  
  print(ans)
  
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"}]