#!/usr/bin/env PyPy3

from collections import Counter, defaultdict, deque
import itertools
import re
import math
from functools import reduce
import operator
import bisect
from heapq import *
import functools

mod=10 ** 9 + 7

import sys
input = sys.stdin.readline
class Bit:#0-indexed を 1-indexedに変換するので、0-indexedを入れる
    def __init__(self, n):
        self.size = n + 1
        self.tree = [0] * (n + 2)
        self.depth = (n + 1).bit_length()

    def add(self, i, x):
        i += 1
        while i <= self.size:
            self.tree[i] += x
            i += i & -i
 
    def sum(self, i):
        i += 1
        s = 0
        while i > 0:
            s += self.tree[i]
            i -= i & -i
        return s
 
    def lower_bound(self, x):
        """ 累積和がx以上になる最小のindexと、その直前までの累積和 """
        sum_ = 0
        pos = 0
        for i in range(self.depth, -1, -1):
            k = pos + (1 << i)
            if k <= self.size and sum_ + self.tree[k] < x:
                sum_ += self.tree[k]
                pos += 1 << i
        return pos, sum_
n = int(input())
s = input().rstrip()
dele = [0,ord('C') - ord('A'),ord('G') - ord('A'),ord('T') - ord('A')]
li = [0] * 26
for ss in s:
    li[ord(ss) - ord('A')] += 1
ans = 0
bit = Bit(n)
for i in range(n):
    bit.add(i,1)
while li[dele[0]] + li[dele[1]] + li[dele[2]] + li[dele[3]]:
    c = li[dele[0]] + li[dele[1]] + li[dele[2]] + li[dele[3]]
    idx,_ = bit.lower_bound(c)
    li[ord(s[idx]) - ord('A')] -= 1
    bit.add(idx,-1)
    for i in range(4):
        dele[i] -= li[ord(s[idx]) - ord('A')]
        dele[i] %= 26
    ans += 1
print(ans)