from collections import Counter


def main():
    n = int(input())
    S = input()
    S_counter = Counter(S)

    a_idx = S.rfind("A")
    g_idx = S.rfind("G")
    c_idx = S.rfind("C")
    t_idx = S.rfind("T")
    most_far_idx = max(a_idx, g_idx, c_idx, t_idx)
    if most_far_idx == -1:
        print(0)
        return

    print(most_far_idx+1)


if __name__ == "__main__":
    main()