import sys,random,bisect
from collections import deque,defaultdict
import heapq
from itertools import permutations
from math import gcd

input = lambda :sys.stdin.readline().rstrip()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

N = int(input())
S = input()

res = 0
for i in range(N):
    if S[i] in "AGCT":
        res = i+1
print(res)