use std::collections::*;

fn main() {
    let n = read::<usize>();
    let t = read_vec::<usize>();
    let mut cands = vec![];
    for i in 0..12 {
        let d = vec![i, i + 2, i + 4, i + 5, i + 7, i + 9, i + 11]
            .into_iter()
            .map(|x| x % 12)
            .collect::<HashSet<_>>();
        if t.iter().all(|&x| d.contains(&x)) {
            cands.push(i);
        }
    }
    if cands.len() != 1 {
        println!("-1");
        return;
    }
    println!("{}", cands[0]);
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}