import re


def main():
    _ = int(input())
    s = input()

    if all((c not in s) for c in "AGCT"):
        exit(print(0))
    # if re.search(r'[AGCT]+', s) is None:
    #     print(0)

    res_iter = re.finditer(r"[AGCT]", s)
    print(max([x.start() + 1 for x in res_iter]))


if __name__ == "__main__":
    main()