// -*- coding:utf-8-unix -*-
// #![feature(map_first_last)]
#![allow(dead_code)]
#![allow(unused_imports)]
#![allow(unused_macros)]
// use core::num;
use std::cmp::*;
use std::fmt::*;
use std::hash::*;
use std::iter::FromIterator;
use std::*;
use std::{cmp, collections, fmt, io, iter, ops, str};
const INF: i64 = 1223372036854775807;
const UINF: usize = INF as usize;
const LINF: i64 = 2147483647;
const INF128: i128 = 1223372036854775807000000000000;
// const MOD: i64 = 1000000007;
const MOD: i64 = 998244353;
const UMOD: usize = MOD as usize;
const M_PI: f64 = 3.14159265358979323846;
// const MOD: i64 = INF;

use cmp::Ordering::*;
use std::collections::*;

use std::io::stdin;
use std::io::stdout;
use std::io::Write;

macro_rules! p {
    ($x:expr) => {
        println!("{}", $x);
    };
}

macro_rules! d {
    ($x:expr) => {
        println!("{:?}", $x);
    };
}
#[allow(unused_macros)]
pub mod macros {
    macro_rules! min { ($x: expr) => { $x }; ($x: expr, $($xs: expr),+) => {{ let y = macros::min!($($xs),+); std::cmp::min($x, y) } }}
    macro_rules! max { ($x: expr) => { $x }; ($x: expr, $($xs: expr),+) => {{ let y = macros::max!($($xs),+); std::cmp::max($x, y) } }}
    macro_rules! chmin { ($x: expr, $($xs: expr),+) => {{ let y = macros::min!($($xs),+); if $x > y { $x = y; true } else { false } }}}
    macro_rules! chmax { ($x: expr, $($xs: expr),+) => {{ let y = macros::max!($($xs),+); if $x < y { $x = y; true } else { false } }}}
    macro_rules! multi_vec { ($element: expr; ($len: expr, $($lens: expr),*)) => ( vec![macros::multi_vec![$element; ($($lens),*)]; $len] ); ($element: expr; ($len: expr)) => ( vec![$element; $len] ); }
    macro_rules! multi_box_array { ($element: expr; ($len: expr, $($lens: expr),*)) => ( vec![macros::multi_box_array![$element; ($($lens),*)]; $len].into_boxed_slice() ); ($element: expr; ($len: expr)) => ( vec![$element; $len].into_boxed_slice() ); }
}
use macros::*;
fn main() {
    solve();
}

// use str::Chars;
#[allow(dead_code)]
fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

#[allow(dead_code)]
fn readi() -> (i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    iter.next().unwrap().parse::<i64>().unwrap()
}

#[allow(dead_code)]
fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}
#[allow(dead_code)]
fn read_mat<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
    (0..n).map(|_| read_vec()).collect()
}

#[allow(dead_code)]
fn readii() -> (i64, i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
    )
}

#[allow(dead_code)]
fn readiii() -> (i64, i64, i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
    )
}
#[allow(dead_code)]
fn readuu() -> (usize, usize) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
    )
}

#[allow(dead_code)]
fn readff() -> (f64, f64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<f64>().unwrap(),
        iter.next().unwrap().parse::<f64>().unwrap(),
    )
}

fn readcc() -> (char, char) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<char>().unwrap(),
        iter.next().unwrap().parse::<char>().unwrap(),
    )
}

fn readuuu() -> (usize, usize, usize) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
    )
}
#[allow(dead_code)]
fn readiiii() -> (i64, i64, i64, i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
    )
}

#[allow(dead_code)]
fn readuuuu() -> (usize, usize, usize, usize) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
    )
}

fn solve() {
    let n: usize = read();
    let mut v: Vec<char> = read::<String>().chars().map(|c| c as char).collect();
    let mut cnt_q = 0;
    let mut cnt_a = 0;
    let mut cnt_c = 0;
    for i in 0..n {
        if v[i] == '?' {
            cnt_q += 1;
        }
        if v[i] == 'A' {
            cnt_a += 1;
        }
        if v[i] == 'C' {
            cnt_c += 1;
        }
    }
    let mut res = 0 as i64;
    let mut r = 0;
    let mut ca = cnt_a + cnt_q;
    for i in (0..n).rev() {
        if v[i] == 'C' {
            r += ca;
        }
        if v[i] == 'A' || v[i] == '?' {
            ca -= 1;
        }
    }
    res = max(res, r);
    let mut upper_cnt_c = 0;
    let mut upper_cnt_q = 0;
    for i in (0..n).rev() {
        if v[i] == '?' {
            r -= upper_cnt_c + upper_cnt_q;
            r += cnt_a + cnt_q - 1;
            res = max(res, r);
        }

        if v[i] == '?' {
            cnt_q -= 1;
            upper_cnt_q += 1;
        }
        if v[i] == 'A' {
            cnt_a -= 1;
        }
        if v[i] == 'C' {
            cnt_c -= 1;
            upper_cnt_c += 1;
        }
    }
    println!("{}", res);
}