n = int(input())
s = list(input())
counter = 0
A = s.count('A')
G = s.count('G')
C = s.count('C')
T = s.count('T')
while True:
    c = A+G+C+T
    if c == 0: break
    else:
        counter += 1
        think = s.pop(c-1)
        if think == 'A': A -= 1
        elif think == 'G': G -= 1
        elif think == 'C': C -= 1
        elif think == 'T': T -= 1
print(counter)