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

a = 0
c = 0
h = 0
for s in S:
    if s == "A":a += 1
    elif s == "C":c += 1
    else:h += 1

dat = [0] * N
A = 0
C = 0
H = 0
tmp = 0
for i in range(N):
    if S[i] == "A":
        dat[i] = tmp + (A + 1) * (c - C + h - H)
        A += 1
    elif S[i] == "C":
        tmp += A
        dat[i] = tmp
        C += 1
    else:
        H += 1
        A += 1
        dat[i] = tmp + A * (c - C + h - H)
A = 0
C = 0
H = 0
ans = 0
now = 0
for i in range(N - 1,0,-1):
    if S[i] == "A":
        tmp = dat[i-1] + C + now
        now += C
        A += 1
        if tmp > ans:ans = tmp
    elif S[i] == "C":
        tmp = dat[i-1] + now
        C += 1
        if tmp > ans:ans = tmp
    else:
        tmp = dat[i-1] + now
        C += 1
        H += 1
        if tmp > ans:
            ans = tmp
print(ans)