fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();
    let mut d = String::new();
    std::io::stdin().read_line(&mut d).ok();
    let d = d.trim().chars().collect::<Vec<_>>();

    let target = "AGCT";
    let cnt = d.iter().filter(|&&c| target.contains(c)).count();
    if cnt == 0 {
        println!("0");
        return;
    }
    let ends = d.iter().enumerate().skip(cnt-1).filter(|&(_, &c)| target.contains(c)).map(|(i, _)| i).collect::<Vec<usize>>();
    if ends.is_empty() {
        println!("{}", cnt);
    } else {
        println!("{}", ends[ends.len()-1] + 1);
    }
}