#include <bits/stdc++.h>

#define pb push_back
#define pf push_front
#define mp make_pair
#define sz size
#define ll long long
#define ld long double
#define fs first
#define sc second
#define forn(i, f, t) for(int i = f; i < t; i++)
#define all(x) (x).begin(), (x).end()
#define ins insert

const int INF = 2147483647;
const int MOD = 1000000007;
const ll INF64 = 9223372036854775807;
const ld EPS = 1e-7;

using namespace std;

pair<int, int> q[100001];


vector<char> used;
set<int> d[100001];
vector<int> res;


void dfs(int v){
    used[v] = 1;
    for (auto it = d[v].begin(); it != d[v].end(); it++) if (!used[*it]) dfs(*it);
    res.pb(v);
}


bool check(int n, int m){
    // cout << n << " " << m << "\n";
    forn(i, 0, n) d[i].clear();
    used.clear();
    res.clear();
    used.assign(n, 0);
    forn(i, 0, m) d[q[i].fs].ins(q[i].sc);
    forn(i, 0, n) if (!used[i]) dfs(i);
    reverse(all(res));
    // forn(i, 0, res.sz()) cout << res[i] << " "; cout << "\n";
    forn(i, 1, res.sz()) if (d[res[i - 1]].find(res[i]) == d[res[i - 1]].end()) return 0;
    if (res.sz() < n) return 0;
    return 1;
}


int main(){
    int n, m, f, t;
    scanf("%d%d", &n, &m);
    forn(i, 0, n){
        scanf("%d%d", &f, &t);
        q[i] = mp(f - 1, t - 1);
    }
    int l = 1, r = m;
    while (l < r - 1){
        int m = (l + r) / 2;
        if (check(n, m)) r = m;
        else l = m;
    }
    printf("%d\n", check(n, l) ? l : (check(n, r) ? r : -1));
    return 0;
}