#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < n; i++)
#define all(v) v.begin(), v.end()
template <class T, class U>
inline bool chmax(T& a, U b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <class T, class U>
inline bool chmin(T& a, U b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
constexpr int INF = 1000000000;
constexpr ll llINF = 3000000000000000000;
constexpr int mod = 998244353;
const ll half = (mod + 1) / 2;
constexpr double eps = 1e-10;
vector<int> prime_list(int n) {
    vector<int> v(n + 1), res;
    for (int i = n; i >= 2; i--) {
        for (int j = i; j <= n; j += i) v[j] = i;
    }
    for (int i = 2; i <= n; i++) {
        if (v[i] == i) res.push_back(i);
    }
    return res;
}
vector<int> next_divisor(int n) {
    vector<int> v(n + 1);
    for (int i = n; i >= 2; i--) {
        for (int j = i; j <= n; j += i) v[j] = i;
    }
    return v;
}
ll modpow(ll a, ll b, ll m = mod) {
    ll res = 1;
    while (b) {
        if (b & 1) {
            res *= a;
            res %= m;
        }
        a *= a;
        a %= m;
        b >>= 1;
    }
    return res;
}
vector<ll> inv, fact, factinv;
void init_fact(int n) {
    inv.resize(n + 1);
    fact.resize(n + 1);
    factinv.resize(n + 1);
    inv[0] = 0;
    inv[1] = 1;
    fact[0] = 1;
    factinv[0] = 1;
    for (ll i = 1; i <= n; i++) {
        if (i >= 2) inv[i] = mod - ((mod / i) * inv[mod % i] % mod);
        fact[i] = (fact[i - 1] * i) % mod;
        factinv[i] = factinv[i - 1] * inv[i] % mod;
    }
}
ll modinv(ll a, ll m = mod) {
    // gcd(a,m) must be 1
    ll b = m, u = 1, v = 0;
    while (b) {
        ll t = a / b;
        a -= t * b;
        swap(a, b);
        u -= t * v;
        swap(u, v);
    }
    u %= m;
    if (u < 0) u += m;
    return u;
}
ll comb(int a, int b) {
    if (a < b || a < 0 || b < 0) return 0;
    return fact[a] * factinv[a - b] % mod * factinv[b] % mod;
}
void solve() {
    int n, q;
    cin >> n >> q;
    vector<int> a(q), b(q);
    rep(i, q) {
        cin >> a[i] >> b[i];
        a[i]--, b[i]--;
    }
    int ok = q + 1, ng = 0;
    while (ok - ng > 1) {
        int m = (ok + ng) / 2;
        vector<vector<int>> g(n);
        vector<int> in(n);
        vector<bool> vis(n);
        rep(i, m) {
            g[a[i]].push_back(b[i]);
            in[b[i]]++;
        }
        queue<int> que;
        rep(i, n) {
            if (in[i] == 0) que.push(i);
        }
        int res = 0;
        bool cycle = false;
        while (!que.empty()) {
            int p = que.front();
            que.pop();
            vis[p] = true;
            for (int i : g[p]) {
                if (vis[i]) {
                    vis[i] = false;
                    cycle = true;
                    break;
                }
                in[i]--;
                if (in[i] == 0) que.push(i);
            }
            if (cycle) break;
        }
        if (count(all(vis), true) == n)
            ng = m;
        else
            ok = m;
    }
    cout << (ok == q + 1 ? -1 : ok) << endl;
}
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    /*int t;
    cin >> t;
    while (t--)*/
    solve();
}