from collections import Counter


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

    operations = 0
    while any(letter in S for letter in ("A", "G", "C", "T")):
        idx = S_counter["A"] + S_counter["G"] + \
            S_counter["C"] + S_counter["T"] - 1
        removed = S[idx]
        S = S[:idx] + S[idx+1:]
        S_counter[removed] -= 1
        operations += 1

    print(operations)


if __name__ == "__main__":
    main()