def oi(): return int(input())
def os(): return input()
def mi(): return list(map(int, input().split()))

from collections import Counter

input_count = 0
N = oi()
S = os()

scounter = Counter(S)

ind = 0
if "A" in scounter:
    ind = max(S.rindex("A")+1, ind)
if "G" in scounter:
    ind = max(S.rindex("G")+1, ind)
if "C" in scounter:
    ind = max(S.rindex("C")+1, ind)
if "T" in scounter:
    ind = max(S.rindex("T")+1, ind)

print(ind)