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'}
last = 0
for i, c in enumerate(S):
    if c in chars:
        last = i + 1
print(last)