N = int(input())
S = list(input())
cnt = 0

for i in range(N):
    if S[i] in "AGCT":
        cnt += 1

if cnt == 0:
    print(0)
    exit()
ans = 0
while True:
    ans += 1
    p = S.pop(cnt - 1)
    if p in "AGCT":
        cnt -= 1
    if cnt == 0:
        break

print(ans)