pub mod scanner {

    pub struct Scanner {
        buf: Vec<String>,
    }

    impl Scanner {
        pub fn new() -> Self {
            Self { buf: vec![] }
        }

        pub fn new_from(source: &str) -> Self {
            let source = String::from(source);
            let buf = Self::split(source);
            Self { buf }
        }

        pub fn next<T: std::str::FromStr>(&mut self) -> T {
            loop {
                if let Some(x) = self.buf.pop() {
                    return x.parse().ok().expect("");
                }
                let mut source = String::new();
                std::io::stdin().read_line(&mut source).expect("");
                self.buf = Self::split(source);
            }
        }

        fn split(source: String) -> Vec<String> {
            source
                .split_whitespace()
                .rev()
                .map(String::from)
                .collect::<Vec<_>>()
        }
    }
}

use crate::scanner::Scanner;
use std::io::Write;
fn main() {
    let mut scanner = Scanner::new();
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    let t: usize = 1;
    for _ in 0..t {
        solve(&mut scanner, &mut out);
    }
}
fn solve(scanner: &mut Scanner, out: &mut std::io::BufWriter<std::io::StdoutLock>) {
    let n: usize = scanner.next();
    let m: usize = scanner.next();
    let mut outdeg = vec![0isize; n];
    let mut indeg = vec![0isize; n];
    for _ in 0..m {
        let u: usize = scanner.next::<usize>() - 1;
        let v: usize = scanner.next::<usize>() - 1;
        outdeg[u] += 1;
        indeg[v] += 1;
    }
    let mut sum_out = 0;
    let mut sum_in = 0;
    for i in 0..n {
        if indeg[i] <= outdeg[i] {
            sum_out += outdeg[i] - indeg[i];
        } else {
            sum_in += indeg[i] - outdeg[i];
        }
    }
    if sum_out != sum_in {
        writeln!(out, "-1").unwrap();
    } else {
        let ans = (sum_out - 1).max(0);
        writeln!(out, "{}", ans).unwrap();
    }
}