use std::io::Read;

fn main() {
	let mut s = String::new();
	std::io::stdin().read_to_string(&mut s).ok();
	let v: Vec<usize> = s.split_whitespace().flat_map(str::parse).collect();
	for i in 0..v[0] * v[1] {
		if v[i % v[0] + 2] == v[i % v[1] + 2 + v[0]] {
			println!("{}", i + 1);
			return;
		}
	}
	println!("-1")
}