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)-1

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

    A=list(S)
    B=list(S)

    for i in range(len(LIST)):
        if i<=l:
            A[LIST[i]]="A"
        else:
            A[LIST[i]]="C"

        if i<=l+1:
            B[LIST[i]]="A"
        else:
            B[LIST[i]]="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=list(S)
B=list(S)

for i in range(len(LIST)):
    if i<=l:
        A[LIST[i]]="A"
    else:
        A[LIST[i]]="C"

    if i<=l+1:
        B[LIST[i]]="A"
    else:
        B[LIST[i]]="C"

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