#[allow(unused_imports)]
use std::{
    convert::{Infallible, TryFrom, TryInto as _}, fmt::{self, Debug, Display, Formatter,},
    fs::{File}, hash::{Hash, Hasher}, iter::{Product, Sum}, marker::PhantomData,
    ops::{Add, AddAssign, Sub, SubAssign, Div, DivAssign, Mul, MulAssign, Neg, RangeBounds},
    str::FromStr, sync::{atomic::{self, AtomicU32, AtomicU64}, Once},
    collections::{*, btree_set::Range}, mem::{swap},
    cmp::{self, Reverse, Ordering, Eq, PartialEq, PartialOrd},
    thread::LocalKey, f64::consts::PI, time::Instant, cell::RefCell,
    io::{self, stdin, Read, read_to_string, BufWriter, BufReader, stdout, Write},
};
#[allow(unused_imports)]
use proconio::{input, input_interactive, marker::{*}};
#[allow(unused_imports)]
//use rand::{thread_rng, Rng, seq::SliceRandom};
#[allow(unused_imports)]
//use ac_library::{*, ModInt998244353 as mint};

#[allow(dead_code)]
const INF: i64 = 1<<60;
#[allow(dead_code)]
const SI: i32 = 1<<30;
#[allow(dead_code)]
const MOD: i64 = 998244353;
#[allow(dead_code)]
const D: [(usize, usize); 4] = [(1, 0), (0, 1), (!0, 0), (0, !0)];
#[allow(dead_code)]
const D2: [(usize, usize); 8] = [(1, 0), (1, 1), (0, 1), (!0, 1), (!0, 0), (!0, !0), (0, !0), (1, !0)];

//use proconio::fastout;
//#[fastout]
fn main() {
    input!{
        n: usize, q: usize,
        e: [(Usize1, Usize1); q],
    }
    let mut l = 0;
    let mut r = q+1;
    while l+1 < r{
        let m = (l+r)/2;
        let res = scc(n, &e[..m]);
        if res.len() < n{
            r = m;
        } else {
            l = m;
        }
    }
    if r > q{
        println!("-1");
    } else {
        println!("{}", r);
    }
}

pub fn scc(n: usize, e: &[(usize, usize)]) -> Vec<Vec<usize>>{
    let mut ord = Vec::with_capacity(n);
    let mut used = vec![false; n];
    let mut cnt = 0;
    let mut edge = vec![Vec::new(); n];
    let mut rev = vec![Vec::new(); n];
    for &(u, v) in e{
        edge[u].push(v);
        rev[v].push(u);
    }
    for i in 0..n{
        scc_dfs1(i, &edge, &mut used, &mut ord, &mut cnt);
    }
    used = vec![false; n];
    let mut res = Vec::new();
    for &v in ord.iter().rev(){
        if used[v]{continue}
        let mut ret = Vec::new();
        scc_dfs2(v, &rev, &mut used, &mut ret);
        res.push(ret);
    }
    res
}

#[inline]
fn scc_dfs1(p: usize, edge: &Vec<Vec<usize>>, used: &mut Vec<bool>, ord: &mut Vec<usize>, cnt: &mut usize){
    if used[p]{return;}
    used[p] = true;
    for &nex in &edge[p]{
        if used[nex]{continue}
        scc_dfs1(nex, edge, used, ord, cnt);
    }
    ord.push(p);
    *cnt += 1;
}

#[inline]
fn scc_dfs2(p: usize, edge: &Vec<Vec<usize>>, used: &mut Vec<bool>, res: &mut Vec<usize>){
    if used[p]{return;}
    used[p] = true;
    res.push(p);
    for &nex in &edge[p]{
        if used[nex]{continue}
        scc_dfs2(nex, edge, used, res);
    }
}