use std::io::Read;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_to_string(&mut s).ok();
	let mut n = s.split_whitespace().skip(1).flat_map(str::parse::<u8>);
	let mut a = -1;
	for d in 0..12 {
		if n.clone()
			.any(|t| matches!((t + 12 - d) % 12, 1 | 3 | 6 | 8 | 10))
		{
			continue;
		}
		if a != -1 {
			a = -1;
			break;
		}
		a = d as i8;
	}
	println!("{a}")
}