import sys
input = sys.stdin.readline

N=int(input())
S=input().strip()
cnt=0
for s in S:
    if s in "AGCT":
        cnt+=1

L=list(S[:cnt])
R=list(S[cnt:])[::-1]
ans=0
while cnt>0:
    ans+=1
    s = L.pop()
    if s in "AGCT":
        cnt-=1
    else:
        L.append(R.pop())
print(ans)