"""

あらかじめ反転
1色にできるか？

"""

from sys import stdin


N = n = int(stdin.readline())

s = list(stdin.readline()[:-1])

oa = ord("A")
agct = [ord("A")-oa , ord("G")-oa, ord("C")-oa, ord("T")-oa]
for i in range(N):
    s[i] = ord(s[i])-oa

ans = 0
for loop in range(n):

    c1 = 0
    for i in range(len(s)):
        if s[i] in agct:
            c1 += 1

    if c1 == 0:
        break
    ans += 1

    t = []
    c2 = 0
    CC = s[c1-1]
    
    for i in range(len(s)):
        if i != c1-1:
            t.append(s[i])
            if s[i] == CC:
                c2 += 1

    for i in range(len(t)):
        t[i] += c2
        t[i] %= 26

    s = t
print (ans)