import sys
import pypyjit
import itertools
import heapq
import math
from collections import deque, defaultdict, Counter
import string
import random

input = sys.stdin.readline
sys.setrecursionlimit(10 ** 6)
pypyjit.set_param('max_unroll_recursion=-1')


N = int(input())
S = input()[:-1]

chars = {'A', 'G', 'C', 'T'}
cnt = 0
first = None
last = None
for i, c in enumerate(S):
    if c in chars:
        if first is None:
            first = i
        last = i
        cnt += 1
if first is None:
    print(0)
else:
    print(last - min(first, cnt - 1) + 1)