n = int(input())
s = list(input())
counter = 0
start = s.count('A') + s.count('G') + s.count('C') + s.count('T')
checklist = ['A','G','C','T']
while start != 0:
    counter += 1
    a = s.pop(start-1)
    if a in checklist:
        start -= 1
print(counter)