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