Buggy python submission (verdict: Time Limit Exceeded):
import sys
import copy
import math
import bisect
import pprint
import bisect
from functools import reduce
from copy import deepcopy
from collections import deque
from decimal import *

import math
from functools import reduce

def gcd(*numbers):
    return reduce(math.gcd, numbers)

def gcd_list(numbers):
    return reduce(math.gcd, numbers)

if __name__ == '__main__':
    n = int(input())

    a = input()
    listr = [0]*n
    listg = [0]*n
    listb = [0]*n
    r =a.count("R")
    g =a.count("G")
    b =a.count("B")

    count = 0

    for i in range(n):

        if a[i] == "R":
            r -=1
        if a[i] == "G":
            g -=1
        if a[i] == "B":
            b -=1
        listr[i] =r
        listb[i] =b
        listg[i] =g

    for i in range(n-2):
        for j in range(i+1,n-1):
            if a[i] != a[j]:
                if a[i] =="R" and a[j] =="G":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue
                if a[i] =="R" and a[j] =="B":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="B":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="R":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="G":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="R":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue


    print(count)

Fixed submission:
import sys
import copy
import math
import bisect
import pprint
import bisect
from functools import reduce
from copy import deepcopy
from collections import deque
from decimal import *

import math
from functools import reduce

def gcd(*numbers):
    return reduce(math.gcd, numbers)

def gcd_list(numbers):
    return reduce(math.gcd, numbers)

if __name__ == '__main__':
    n = int(input())

    a = input()
    listr = [0]*n
    listg = [0]*n
    listb = [0]*n
    r =a.count("R")
    g =a.count("G")
    b =a.count("B")

    count = 0

    for i in range(n):

        if a[i] == "R":
            r -=1
        if a[i] == "G":
            g -=1
        if a[i] == "B":
            b -=1
        listr[i] =r
        listb[i] =b
        listg[i] =g

    for i in range(n-2):
        for j in range(i+1,n-1):
            if a[i] != a[j]:
                if a[i] =="R" and a[j] =="G":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue
                if a[i] =="R" and a[j] =="B":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="B":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="R":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="G":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="R":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue


    print(count)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
import copy
import math
import bisect
import pprint
import bisect
from functools import reduce
from copy import deepcopy
from collections import deque
from decimal import *

import math
from functools import reduce

def gcd(*numbers):
    return reduce(math.gcd, numbers)

def gcd_list(numbers):
    return reduce(math.gcd, numbers)

if __name__ == '__main__':
    n = int(input())

    a = input()
    listr = [0]*n
    listg = [0]*n
    listb = [0]*n
    r =0
    g =0
    b =0

    count = 0

    for i in range(n):

        if a[i] == "R":
            r +=1
        if a[i] == "G":
            g +=1
        if a[i] == "B":
            b +=1
        listr[i] =r
        listb[i] =b
        listg[i] =g

    for i in range(n):

        if a[i] == "R":
            r -=1
        if a[i] == "G":
            g -=1
        if a[i] == "B":
            b -=1
        listr[i] =r
        listb[i] =b
        listg[i] =g

    for i in range(n-2):
        for j in range(i+1,n-1):
            if a[i] != a[j]:
                if a[i] =="R" and a[j] =="G":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue
                if a[i] =="R" and a[j] =="B":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="B":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="R":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="G":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="R":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue


    print(count)

Fixed submission:
import sys
import copy
import math
import bisect
import pprint
import bisect
from functools import reduce
from copy import deepcopy
from collections import deque
from decimal import *

import math
from functools import reduce

def gcd(*numbers):
    return reduce(math.gcd, numbers)

def gcd_list(numbers):
    return reduce(math.gcd, numbers)

if __name__ == '__main__':
    n = int(input())

    a = input()
    listr = [0]*n
    listg = [0]*n
    listb = [0]*n
    r =a.count("R")
    g =a.count("G")
    b =a.count("B")

    count = 0

    for i in range(n):

        if a[i] == "R":
            r -=1
        if a[i] == "G":
            g -=1
        if a[i] == "B":
            b -=1
        listr[i] =r
        listb[i] =b
        listg[i] =g

    for i in range(n-2):
        for j in range(i+1,n-1):
            if a[i] != a[j]:
                if a[i] =="R" and a[j] =="G":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue
                if a[i] =="R" and a[j] =="B":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="B":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue
                if a[i] =="G" and a[j] =="R":
                    count += listb[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="B" and listb[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="G":
                    count += listr[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="R" and listr[j] >0:
                        count -=1
                    continue

                if a[i] =="B" and a[j] =="R":
                    count += listg[j]
                    if (j-i)+j < len(a) and a[(j-i)+j] =="G" and listg[j] >0:
                        count -=1
                    continue


    print(count)