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()
  
  RGB = [[] for i in range(3)]
  for i in range(N):
      if S[i] == "R":
          RGB[0].append(i)
      elif S[i] == "G":
          RGB[1].append(i)
      elif S[i] == "B":
          RGB[2].append(i)
  
  ans = 0
  for n in range(3):
      for i in range(len(RGB[n])):
          j_start = 0
          while RGB[n][i] > RGB[(n + 1) % 3][j_start]:
              if j_start + 1 < len(RGB[(n + 1) % 3]):
                  j_start += 1
              else:
                  break
          for j in range(j_start, len(RGB[(n + 1) % 3])):
              k_start = 0
              while RGB[(n + 1) % 3][j] > RGB[(n + 2) % 3][k_start]:
                  if k_start + 1 < len(RGB[(n + 2) % 3]):
                      k_start += 1
                  else:
                      break
              for k in range(len(RGB[(n + 2) % 3])):
                  if (
                      RGB[n][i] < RGB[(n + 1) % 3][j] < RGB[(n + 2) % 3][k]
                      and RGB[(n + 1) % 3][j] - RGB[n][i]
                      != RGB[(n + 2) % 3][k] - RGB[(n + 1) % 3][j]
                  ):
                      ans += 1
  
          j_start = 0
          while RGB[n][i] > RGB[(n + 2) % 3][j_start]:
              if j_start + 1 < len(RGB[(n + 2) % 3]):
                  j_start += 1
              else:
                  break
          for j in range(len(RGB[(n + 2) % 3])):
              k_start = 0
              while RGB[(n + 2) % 3][j] > RGB[(n + 1) % 3][k_start]:
                  if k_start + 1 < len(RGB[(n + 1) % 3]):
                      k_start += 1
                  else:
                      break
              for k in range(len(RGB[(n + 1) % 3])):
                  if (
                      RGB[n][i] < RGB[(n + 2) % 3][j] < RGB[(n + 1) % 3][k]
                      and RGB[(n + 2) % 3][j] - RGB[n][i]
                      != RGB[(n + 1) % 3][k] - RGB[(n + 2) % 3][j]
                  ):
                      ans += 1
  
  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"}]