Buggy python submission (verdict: Time Limit Exceeded):
# abc162_d.py
import sys
import time
import unittest
from io import StringIO
import itertools


def resolve():
    N = int(input())
    S = input()
    sum = 0
    for i, j, k in itertools.combinations(range(N), 3):
        if S[i] != S[j] and S[i] != S[k] and S[j] != S[k] and j - i != k - j:
            sum += 1
    print(sum)


class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)

    def assertTLE(self, input):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin

    # def test_入力例_n(self):
    #     input = """9999992"""
    #     self.assertTLE(input)

    def setUp(self):
        self.startTime = time.time()

    def tearDown(self):
        t = time.time() - self.startTime
        print('%s: %.3f' % (self.id(), t), flush=True)

    def test_入力例_1(self):
        input = """4
RRGB"""
        output = """1"""
        self.assertIO(input, output)

    def test_入力例_2(self):
        input = """39
RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB"""
        output = """1800"""
        self.assertIO(input, output)


if __name__ == "__main__":
    if "IS_LOCAL" in locals():
        suite = unittest.TestLoader().loadTestsFromTestCase(TestClass)
        unittest.TextTestRunner(verbosity=0).run(suite)
    else:
        resolve()


Fixed submission:
# abc162_d.py
import sys
import time
import unittest
from io import StringIO


def resolve():
    int(input())
    S = input()
    r = S.count("R")
    g = S.count("G")
    b = S.count("B")
    sum = r * g * b
    for i in range(len(S)):
        for j in range(i + 1, len(S)):
            k = j + (j - i)
            if k < len(S) and S[i] != S[j] and S[i] != S[k] and S[j] != S[k]:
                sum -= 1
    print(sum)


class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)

    def checkTLE(self, input):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin

    def debugIO(self, input):
        stdin = sys.stdin
        sys.stdin = StringIO(input)
        resolve()
        sys.stdin = stdin

    def setUp(self):
        self.startTime = time.time()

    def tearDown(self):
        t = time.time() - self.startTime
        print('%s: %.3f' % (self.id(), t), flush=True)

    def test_入力例_1(self):
        input = """4
RRGB"""
        output = """1"""
        self.assertIO(input, output)

    def test_入力例_2(self):
        input = """39
RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB"""
        output = """1800"""
        self.assertIO(input, output)


if __name__ == "__main__":
    if "IS_LOCAL" in locals():
        suite = unittest.TestLoader().loadTestsFromTestCase(TestClass)
        unittest.TextTestRunner(verbosity=0).run(suite)
    else:
        resolve()