use std::io;
//use std::str::FromStr;
/*
const LOW_CASE_ALPHABET: [char; 26] = [
    'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
    't', 'u', 'v', 'w', 'x', 'y', 'z',
];
*/

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);
}
/*
pub fn ctoi(c: char) -> isize {
    let n = c as isize - '0' as isize;

    return n;
}

pub fn stoi(c: String) -> isize {
    let n = c.parse::<isize>().unwrap();

    return n;
}

pub fn find_char(sentence: Vec<char>, target: char, start: isize) -> isize {
    for _i in start as usize..sentence.len() {
        if sentence[_i] == target {
            return _i as isize;
        }
    }
    return -1;
}

pub fn hex_to_binary_2(hex_string: String, x: u32) -> Option<String> {
    match i64::from_str_radix(&hex_string, x) {
        Ok(decimal) => Some(format!("{}", decimal)),
        Err(_) => None,
    }
}

pub fn hex_to_binary(hex_string: &str, x: u32) -> Option<String> {
    match i64::from_str_radix(hex_string, x) {
        Ok(decimal) => Some(format!("{}", decimal)),
        Err(_) => None,
    }
}

pub fn my_floor(x: f64) -> f64 {
    let rounded = (x * 1000.0).floor() / 1000.0;

    rounded
}

pub fn upper_lower(x: char) -> char {
    let result: char;

    if x.is_uppercase() {
        result = x.to_ascii_lowercase();
    } else if x.is_lowercase() {
        result = x.to_ascii_uppercase();
    } else {
        result = x;
    }

    result
}

pub fn maxf_2(x: f64, y: f64) -> f64 {
    let result;

    if x >= y {
        result = x;
    } else {
        result = y;
    }

    result
}

pub fn max_2(x: isize, y: isize) -> isize {
    let result;

    if x >= y {
        result = x;
    } else {
        result = y;
    }

    result
}

pub fn max_u_2(x: usize, y: usize) -> usize {
    let result;

    if x >= y {
        result = x;
    } else {
        result = y;
    }

    result
}

pub fn min_u_2(x: usize, y: usize) -> usize {
    let result;

    if x <= y {
        result = x;
    } else {
        result = y;
    }

    result
}

pub fn min_2(x: isize, y: isize) -> isize {
    let result;

    if x >= y {
        result = y;
    } else {
        result = x;
    }

    result
}

pub fn min_2_i32(x: i32, y: i32) -> i32 {
    let result;

    if x >= y {
        result = y;
    } else {
        result = x;
    }

    result
}

pub fn max_3(x: isize, y: isize, z: isize) -> isize {
    let result;
    if x >= y && y >= z {
        result = x;
    } else if x >= z && z >= y {
        result = x;
    } else if y >= x && x >= z {
        result = y;
    } else if y >= z && z >= x {
        result = y;
    } else if z >= x && x >= y {
        result = z;
    } else {
        result = z;
    }

    result
}

pub fn max_u_3(x: usize, y: usize, z: usize) -> usize {
    let result;
    if x >= y && y >= z {
        result = x;
    } else if x >= z && z >= y {
        result = x;
    } else if y >= x && x >= z {
        result = y;
    } else if y >= z && z >= x {
        result = y;
    } else if z >= x && x >= y {
        result = z;
    } else {
        result = z;
    }

    result
}

pub fn min_3(x: isize, y: isize, z: isize) -> isize {
    let result;
    if x <= y && y <= z {
        result = x;
    } else if x <= z && z <= y {
        result = x;
    } else if y <= x && x <= z {
        result = y;
    } else if y <= z && z <= x {
        result = y;
    } else if z <= x && x <= y {
        result = z;
    } else {
        result = z;
    }

    result
}

pub fn min_u_3(x: usize, y: usize, z: usize) -> usize {
    let result;
    if x <= y && y <= z {
        result = x;
    } else if x <= z && z <= y {
        result = x;
    } else if y <= x && x <= z {
        result = y;
    } else if y <= z && z <= x {
        result = y;
    } else if z <= x && x <= y {
        result = z;
    } else {
        result = z;
    }

    result
}

pub fn read_text() -> String {
    let mut word = String::new();
    std::io::stdin().read_line(&mut word).ok();
    let answer = word.trim().to_string();

    answer
}

pub fn read_array() -> Vec<char> {
    let array: String = read();
    let chars: Vec<char> = array.trim().chars().collect::<Vec<char>>();
    chars
}

pub fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}
*/