n = int(input())
s = list(input())
v = 0
hind = []
for i in range(n):
	if s[i] == '?':
		v += 1
		hind.append(i)

nowlft = 0
tmp = 0
for i in range(n):
	if s[i] == '?' or s[i] == 'A':
		nowlft += 1
	elif s[i] == 'C':
		tmp += nowlft

rui = [0] * n
ruil = [0] * n
for i in range(n):
	if s[i] == 'C':
		rui[i] += 1
	if s[i] == 'A':
		ruil[i] += 1

for i in range(n-2,-1,-1):
	rui[i] += rui[i+1]

for i in range(n-1):
	ruil[i+1] += ruil[i]

ans = tmp
m = len(hind)
rgtc = 0
lftc = m
#print(rui)
#print(ruil)
for i in range(m-1,-1,-1):
	# s[i] : "(" -> ")"
	tmp -= rui[hind[i]]
	tmp -= rgtc
	lftc -= 1
	rgtc += 1
	tmp += lftc
	tmp += ruil[hind[i]]
	ans = max(ans, tmp)

print(ans)