N = int(input())
S = input()

ans=0

pos=[0]*N
c=0
for i in range(N):
    if S[i]=="A" or S[i]=="G" or S[i]=="C" or S[i]=="T":
        pos[i]=1
        c+=1

if c>0:
    ans=0
    kesu=c-1
    moto=c-1
    next=c-1
    for i in range(N):
#        print(next)
        ans+=1
        if pos[next]==1:
            moto-=1
            next=moto
            if moto<0:
                break
        else:
            kesu+=1
            next=kesu
            if kesu>(N-1):
                break
    print(ans)

else:
    print(0)