from collections import defaultdict
import sys
readline=sys.stdin.readline
write=sys.stdout.write

N=int(readline())
S=readline().rstrip()
cnt=sum(S.count(s) for s in "AGCT")
if cnt==0:
    ans=0
else:
    l,r=cnt-1,cnt
    cur=cnt-1
    while cnt:
        if S[cur] in "AGCT":
            cnt-=1
            l-=1
            cur=l
        else:
            cur=r
            r+=1
    ans=r
print(ans)