import itertools as iter
import collections as coll
import heapq as hq
import bisect as bis
from decimal import Decimal as dec
from functools import cmp_to_key
import math
import sys
#import pypyjit
#pypyjit.set_param('max_unroll_recursion=-1')
sys.setrecursionlimit(10 ** 6)
inp = sys.stdin.readline
input = lambda : inp().rstrip()
getN = lambda : int(inp())
getNs = lambda : map(int, inp().split())
getList = lambda :list(map(int, inp().split()))
getStrs = lambda n : [input() for _ in [0] * n]
def yexit(): print("Yes"); exit(0)
def nexit(): print("No"); exit(0)
pi = 3.141592653589793
mod = 1000000007
MOD = 998244353
INF = 4611686018427387903
dx = [1, 0, -1, 0];	dy = [0, 1, 0, -1]


"""
Main Code
"""

n = getN()
s = input()
lis = []

cnt = 0
for c in s:
	if(c in "AGCT"):
		cnt += 1
	lis.append(c)

if(cnt == 0):
	print(0)
	exit(0)

que1 = coll.deque(lis[:(cnt - 1)])
que2 = coll.deque(lis[(cnt - 1):])
while(cnt > 0):
	#print(que1)
	#print(que2)
	c = que2.popleft()
	cnt -= (c in "AGCT")
	if(c in "AGCT" and len(que1) > 0):
		d = que1.pop()
		que2.appendleft(d)
print(n - len(que1) - len(que2))