n = int(input())
s = list(input())
while s.count('A')+s.count('G')+s.count('C')+s.count('T'):
    cnt = s.count('A')+s.count('G')+s.count('C')+s.count('T')
    t = []
    for i,c in enumerate(s):
        if i!=cnt-1:
            t.append(c)
    r = t.count(s[cnt-1])
    for i in range(len(t)):
        t[i] = chr((ord(t[i])-65+r)%26+65)
    s = t
print(n-len(s))