use std::io::Read;

fn main() {
    const DIVIDER: u128 = 1_000_000_000_000_000_000;
    let mut all_data = String::new();
    std::io::stdin().read_to_string(&mut all_data).ok();
    let all_data: Vec<&str> = all_data.trim().split('\n').map(|a| a.trim()).collect();
    let n: usize = all_data.iter().next().unwrap().parse::<usize>().unwrap();
    let p: Vec<u128> = all_data.iter().skip(1).next().unwrap().split_whitespace().map(|a| a.parse::<u128>().unwrap()).collect();
    let mut result: Vec<u128> = vec![p[0]];
    p.iter().skip(1).for_each(|a| {
        let old = result.clone().to_owned();
        result.clear();
        let mut carry_over = 0;
        old.iter().for_each(|r| {
            let rest = (r * *a + carry_over) % DIVIDER;
            carry_over = (r * *a + carry_over) / DIVIDER;
            result.push(rest);
        });
        if carry_over > 0 {
            result.push(carry_over);
        }
    });
    let mut p: u128 = result.iter().map(|a| a.to_string().chars().map(|b| b.to_string().parse::<u128>().unwrap()).sum::<u128>()).sum();

    while p >= 10 {
        let p_str: String = p.to_string();
        p = p_str.chars().map(|a| a.to_string().parse::<u128>().unwrap()).sum();
    }
    println!("{}", p);
}