use std::io::*;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let n: usize = itr.next().unwrap().parse().unwrap();
    let a: Vec<usize> = (0..n)
        .map(|_| itr.next().unwrap().parse().unwrap())
        .collect();

    let b = [0, 2, 4, 5, 7, 9, 11].to_vec();
    let mut ans: i32 = -1;
    for d in 0..12 {
        let mut ok = true;
        for i in 0..n {
            let mut ng = true;
            for j in 0..7 {
                if (d + b[j]) % 12 == a[i] {
                    ng = false;
                }
            }
            if ng {
                ok = false;
            }
        }
        if ok {
            if ans == -1 {
                ans = d as i32;
            } else {
                println!("-1");
                return;
            }
        }
    }
    println!("{}", ans);
}