fn main() {
    let n: usize = {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        buf.trim_end().parse().unwrap()
    };
    let t: Vec<i32> = {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        let iter = buf.split_whitespace();
        iter.map(|x| x.parse().unwrap()).collect()
    };

    let mut ans = -1;
    for d in 0..12 {
        let mut is_true = true;
        for i in 0..n {
            let mut is_included = false;
            for j in vec![0, 2, 4, 5, 7, 9, 11] {
                if t[i] == (d + j) % 12 {
                    is_included = true;
                }
            }
            if !is_included {
                is_true = false;
            }
        }
        if is_true && ans < 0 {
            ans = d;
        } else if is_true {
            ans = -1;
            break;
        }
    }
    println!("{}", ans);
}