use std::io;

fn main() {
    let mut nn = String::new();
    io::stdin().read_line(&mut nn);
    let n = match nn.trim().parse() {
        Ok(num) => num,
        Err(_) => 0
    };
    let mut ss = String::new();
    io::stdin().read_line(&mut ss);
    let s: Vec<char> = ss.chars().collect();
    let mut ans = 0;
    for i in 0..n {
        let c = s[i];
        if c == 'A' || c == 'G' || c == 'C' || c == 'T' {
            ans = i + 1;
        }
    }
    println!("{}", ans);
}