#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;

#define rep(i, n) for(ll i = 0; i < n; i++)
#define rep2(i, m, n) for(ll i = m; i <= n; i++)
#define rrep(i, m, n) for(ll i = m; i >= n; i--)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define MAX(x) *max_element(all(x))
#define MIN(x) *min_element(all(x))
#define SZ(a) ((ll)(a).size())
#define bit(n, k) ((n >> k) & 1)
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const int dx[4] = { -1, 1, 0, 0 };
const int dy[4] = { 0, 0, 1, -1 };
const int inf = 1e9 + 1;
const ll INF = 1e18;
const double pi = acos(-1);

using Graph = vector<vector<ll>>;
using REV_PQ = priority_queue<ll, vector<ll>, greater<ll>>;
using PQ = priority_queue<ll>;

//typedef atcoder::modint998244353 mint;
typedef atcoder::modint1000000007 mint;

const int SIZE = 200010;

ll powll(ll n, ll x) {
    // return n ^ x;

    ll ret = 1;
    rep(i, x) ret *= n;

    return ret;
}

/*struct Edge { ll u, v, cost; };

bool comp(const Edge& e1, const Edge& e2) {
    return e1.cost < e2.cost;
}*/

void OutputYesNo(bool val) {

    if (val) cout << "Yes" << endl;
    else cout << "No" << endl;
}
void OutputTakahashiAoki(bool val) {

    if (val) cout << "Takahashi" << endl;
    else cout << "Aoki" << endl;
}

void OutPutInteger(ll x) { cout << x << endl; }
void OutPutString(string x) { cout << x << endl; }

int n;
int a[SIZE];
int b[SIZE];

bool Toposo(const Graph& G, const Graph& G_rev) {

    vector<int> ret;
    vector<int> index(n), index_rev(n);

    queue<int> q;
    rep(i, n) {
        for (auto v : G[i]) {
            index[v]++;
        }    
    }

    rep(i, n) {
        for (auto v : G_rev[i]) {
            index_rev[v]++;
        }
    }
    rep(i, n) if (index_rev[i] > 0 and index[i] == 0) q.push(i);
    int sz = 0;
    rep(i, n) if (index_rev[i] + index[i] > 0) sz++;

    while (!q.empty()) {
        int cur = q.front(); 
        ret.push_back(cur);
        q.pop();
        for (auto v : G[cur]) {
            index[v]--;
            if (index[v] == 0) {
                q.push(v);
            }
        }
    }

    if (SZ(ret) != sz) return true;
    return false;

}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int q; cin >> n >> q;
    rep(i, q) {
        cin >> a[i] >> b[i];
        a[i]--, b[i]--;
    }

    int mi = 1, ma = q;
    bool ok = false;

    while (ma - mi > 1) {
        int mid = (mi + ma) / 2;

        Graph G(n), G_rev(n);

        rep(i, mid) {
            G[a[i]].push_back(b[i]);
            G_rev[b[i]].push_back(a[i]);
        }

        if (Toposo(G, G_rev)) {
            ma = mid;
            ok = true;
        }
        else mi = mid;

    }

    if (ok) OutPutInteger(mi +1);
    else OutPutInteger(-1);

 }