import sys,collections as cl,bisect as bs
sys.setrecursionlimit(100000)
input = sys.stdin.readline
mod = 10**9+7
Max = sys.maxsize
def l(): #intのlist
  return list(map(int,input().split()))
def m(): #複数文字
  return map(int,input().split())
def onem(): #Nとかの取得
  return int(input())
def s(x): #圧縮
  a = []
  if len(x) == 0:
    return []
  aa = x[0]
  su = 1
  for i in range(len(x)-1):
    if aa != x[i+1]:
      a.append([aa,su])
      aa = x[i+1]
      su = 1
    else:
      su += 1
  a.append([aa,su])
  return a
def jo(x): #listをスペースごとに分ける
  return " ".join(map(str,x))
def max2(x): #他のときもどうように作成可能
  return max(map(max,x))
def In(x,a): #aがリスト(sorted)
    k = bs.bisect_left(a,x)
    if k != len(a) and a[k] ==  x:
        return True
    else:
        return False

def pow_k(x, n):
    ans = 1
    while n:
        if n % 2:
            ans *= x
        x *= x
        n >>= 1
    return ans

import fractions
from functools import reduce
def gcd(*numbers):
    return reduce(fractions.gcd, numbers)

def gcd_list(numbers):
    return reduce(fractions.gcd, numbers)
def lcm_base(x, y):
    return (x * y) // fractions.gcd(x, y)

def lcm(*numbers):
    return reduce(lcm_base, numbers, 1)

def lcm_list(numbers):
    return reduce(lcm_base, numbers, 1)
"""
n = onem()

s = input()[:-1]

R = [0 for _ in range(n)]
B = [0 for _ in range(n)]
G = [0 for _ in range(n)]

co = 0

if s[0] == "R":
  R[0] = 1
elif s[0] == "G":
  G[0] = 1
else:
  B[0] = 1

for i in range(1,n):
  if s[i] == "R":
    R[i] += 1
  elif s[i] == "G":
    G[i] += 1
  else:
    B[i] += 1
  R[i] += R[i-1]
  G[i] += G[i-1]
  B[i] += B[i-1]

for i in range(1,n-1):
  if s[i] == "G":
    for j in range(i):
      if s[j] == "R":
        co += B[-1] - B[i] - (1 if (j + (j - i) < n) and s[j + (j - i)] == "B" else 0)
print(co)

"""
n = onem()



a = [1, 9, 30, 76, 141, 267, 400, 624, 885, 1249, 1590, 2208, 2689, 3411, 4248, 5248, 6081, 7485, 8530, 10248, 11889, 13687, 15228, 17988, 20053, 22569, 25242, 28588, 31053, 35463, 38284, 42540, 46581, 50893, 55362, 61824, 65857, 71247, 76884, 84388, 89349, 97881, 103342, 111528, 120141, 128047, 134580, 146316, 154177, 164817, 174438, 185836, 194157, 207927, 218812, 233268, 245277, 257857, 268182, 288216, 299257, 313635, 330204, 347836, 362973, 383709, 397042, 416448, 434025, 456967, 471948, 499740, 515581, 536073, 559758, 583960, 604833, 633651, 652216, 683712, 709065, 734233, 754734, 793188, 818917, 846603, 874512, 909496, 933081, 977145, 1006126, 1041504, 1073385, 1106467, 1138536, 1187112, 1215145, 1255101, 1295142, 1342852, 1373253, 1422195, 1453816, 1502376, 1553361, 1595437, 1629570, 1691292, 1726717, 1782111, 1827492, 1887772, 1925853, 1986837, 2033674, 2089776, 2145333, 2197483, 2246640, 2332104, 2379085, 2434833, 2490534, 2554600, 2609625, 2693919, 2742052, 2813988, 2875245, 2952085, 3003306, 3096024, 3157249, 3224511, 3306240, 3388576, 3444609, 3533637, 3591322, 3693924, 3767085, 3842623, 3912324, 4027884, 4102093, 4181949, 4270422, 4361548, 4427853, 4548003, 4616104, 4718640, 4812789, 4918561, 5003286, 5131848, 5205481, 5299011, 5392008, 5521384, 5610705, 5739009, 5818390, 5930196, 6052893, 6156139, 6239472, 6402720, 6493681, 6623853, 6741078, 6864016, 6953457, 7094451, 7215016, 7359936, 7475145, 7593865, 7689630, 7886244, 7984165, 8130747, 8253888, 8403448, 8523897, 8684853, 8802826, 8949612, 9105537, 9267595, 9376656, 9574704, 9686065, 9827097, 9997134, 10174780, 10290813, 10493367, 10611772,10813692]


print(a[n-1])



Measured memory_bytes: 77108