n = int(input())
s = input()
AGCT = ["A","G","C","T"]

tmp = [True]*(len(s))
cnt = 0

for i,v in enumerate(s):
    if v in AGCT:
        cnt+=1

ans = 0
while cnt>0:
    tmp2 = cnt-1
    flag = True

    while flag:
        if tmp[tmp2]:
            tmp[tmp2]=False
            if s[tmp2] in AGCT:
                cnt-=1
            ans+=1
            flag = False
            break
        else:
            tmp2+=1

print(ans)