import sys
input = sys.stdin.readline

N=int(input())
S=input().strip()

LIST=[]

for i in range(N):
    if S[i]=="?":
        LIST.append(i)

def score(A):
    ANS=0
    now=0

    for x in A:
        if x=="A":
            now+=1
        else:
            ANS+=now
    return ANS


MIN=0
MAX=len(LIST)

while MAX>MIN+1:
    l=(MIN+MAX)//2

    A=[]
    B=[]

    for i in range(len(S)):
        s=S[i]
        if s!="?":
            A.append(s)
            B.append(s)
        else:
            if i<=l:
                A.append("A")
            else:
                A.append("C")

            if i<=l+1:
                B.append("A")
            else:
                B.append("C")

    if score(A)<score(B):
        MIN=l+1
    elif score(A)>score(B):
        MAX=l
    else:
        MIN=l
        MAX=l+1

l=MIN
A=[]
B=[]
for i in range(len(S)):
    s=S[i]
    if s!="?":
        A.append(s)
        B.append(s)
    else:
        if i<=l:
            A.append("A")
        else:
            A.append("C")

        if i<=l+1:
            B.append("A")
        else:
            B.append("C")
        
print(max(score(A),score(B)))