fn solve(scanner: &mut Scanner) {
    let n: usize = scanner.next();
    let s: Vec<char> = scanner.next::<String>().chars().collect();
    let s = s
        .iter()
        .map(|&c| {
            if c == 'A' {
                0
            } else if c == 'C' {
                1
            } else {
                2
            }
        })
        .collect::<Vec<usize>>();
    let mut sum = vec![vec![0_usize; n + 1]; 3];
    for (i, &c) in s.iter().enumerate() {
        for j in 0..3 {
            sum[j][i + 1] = sum[j][i];
        }
        sum[c][i + 1] += 1;
    }
    let mut ans = 0;
    for (i, &c) in s.iter().enumerate() {
        if c == 1 || c == 2 {
            ans += sum[0][i];
        }
    }
    let mut now = ans;
    for (i, &c) in s.iter().enumerate() {
        if c == 2 {
            now -= sum[0][i] + sum[2][i];
            now += sum[1][n] - sum[1][i + 1] + sum[2][n] - sum[2][i + 1];
        }
        ans = ans.max(now);
    }
    println!("{}", ans);
}

fn main() {
    let mut scanner = Scanner::new();
    let t: usize = 1;
    for _ in 0..t {
        solve(&mut scanner);
    }
}

pub struct Scanner {
    buf: Vec<String>,
}

impl Scanner {
    fn new() -> Self {
        Self { buf: vec![] }
    }

    fn next<T: std::str::FromStr>(&mut self) -> T {
        loop {
            if let Some(x) = self.buf.pop() {
                return x.parse().ok().expect("");
            }
            let mut source = String::new();
            std::io::stdin().read_line(&mut source).expect("");
            self.buf = Self::split(source);
        }
    }

    fn split(source: String) -> Vec<String> {
        source
            .split_whitespace()
            .rev()
            .map(String::from)
            .collect::<Vec<_>>()
    }
}