fn main() {
    println!("{}", solve());
}

fn solve() -> usize {
    let n = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().parse::<usize>().unwrap()
    };
    let cc: Vec<char> = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().chars().collect()
    };

    for i in (0..n).rev() {
        if ['A', 'G', 'C', 'T'].contains(&cc[i]) {
            return i + 1;
        }
    }

    0
}