n = int(input()); s = input()
a = [0]*n; c = [0]*n; ans = 0
for i in range(n):
    if i>0: a[i] = a[i-1]
    if s[i]!="C": a[i] += 1
    else: ans += a[i]
x = ans
for i in range(n-1,-1,-1):
    if i<n-1: c[i] = c[i+1]
    if s[i]!="A": c[i] += 1
    if s[i]=="?": x += a[i]-c[i]; ans = max(ans,x)
print(ans)