#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
using Vi = vector<int>;
using VVi = vector<Vi>;
using Vl = vector<ll>;
using VVl = vector<Vl>;
using P = pair<int, int>;
using Vp = vector<P>;
using VVp = vector<Vp>;
using Pl = pair<ll, int>;
using Vpl = vector<Pl>;
using VVpl = vector<Vpl>;
using tup = tuple<ll, ll, int>;
using Vt = vector<tuple<int, int, int>>;
using Pll = pair<ll, ll>;
using Vc = vector<char>;
using VVc = vector<Vc>;
template <class U>
using PQmax = priority_queue<U>;
template <class U>
using PQmin = priority_queue<U, vector<U>, greater<U>>;


void tprsort(int u, const VVi& gr, Vi& tpr, Vi& par);
void tprsort(const VVi& gr, Vi& tpr);
ll mpow(ll x, ll n, ll m = 1e9 + 7);
ll comb(int n, int r, const Vl& kai, const Vl& fkai, ll m = 1e9 + 7);
ll gcd(ll a, ll b);
int LCA(const VVi& par, const Vi& depth, int a, int b);
Vi sccResolve(const VVi& gr);
void dijkstra(const VVi& gr, const VVl& cost, Vl& dist, int s);
class unionfind
{
private:
    Vi par;
    Vi siz;
    Vi val;

public:
    unionfind(int N);
    unionfind();
    int root(int v);
    void merge(int a, int b);
    bool same(int a, int b);
    int value(int a);
};
class BIT
{
private:
    Vl bit;
    int siz;

public:
    BIT(int N);
    BIT();
    ll& get(int id);
    void add(int id, ll a, ll m);
    ll sum(int id, ll m);
};

int main()
{
    int N, M;
    cin >> N >> M;
    Vp ed(M);
    for (auto& [a, b] : ed) {
        cin >> a >> b;
        a--;
        b--;
    }
    int mi = 1;
    int ma = M;
    while (mi != ma) {
        VVi gr(N);
        Vi cnt(N, 0);
        int mu = (mi + ma) / 2;
        for (int i = 0; i < mu; i++) {
            auto [a, b] = ed[i];
            gr[a].push_back(b);
            cnt[b]++;
        }
        queue<int> que;
        for (int i = 0; i < N; i++) {
            if (cnt[i] == 0) {
                que.push(i);
            }
        }
        while (!que.empty()) {
            int u = que.front();
            que.pop();
            for (auto v : gr[u]) {
                cnt[v]--;
                if (cnt[v] == 0) {
                    que.push(v);
                }
            }
        }
        bool ok = true;
        for (int i = 0; i < N; i++) {
            if (cnt[i]) {
                ok = false;
            }
        }
        if (!ok) {
            ma = mu;
        } else {
            mi = mu + 1;
        }
    }
    cout << (mi == M ? -1 : mi) << endl;
}
// Library
void tprsort(const VVi& gr, Vi& tpr)
{
    queue<int> que;
    int N = gr.size();
    Vi siz(N, 0);
    for (int i = 0; i < N; i++) {
        for (int j : gr[i]) {
            siz[j]++;
        }
    }
    for (int i = 0; i < N; i++) {
        if (siz[i] == 0) {
            que.push(i);
        }
    }
    while (!que.empty()) {
        int u = que.front();
        que.pop();
        tpr.push_back(u);
        for (int v : gr[u]) {
            siz[v]--;
            if (siz[v] == 0) {
                que.push(v);
            }
        }
    }
    reverse(tpr.begin(), tpr.end());
}
void tprsort(int u, const VVi& gr, Vi& tpr, Vi& par)
{
    // idx[u] = tpr.size();
    /*for (int v : gr[u]) {
        if (par[v] == u || par[v] == -1) {
            par[v] = u;
            tprsort(v, gr, tpr, par);
        }
    }
    tpr.push_back(u);*/
    stack<int> st;
    st.push(u);
    bool vis[2000020];
    for (int i = 0; i <= gr.size(); i++) {
        vis[i] = false;
    }
    while (!st.empty()) {
        int v = st.top();
        if (!vis[v]) {
            vis[v] = true;
            for (int p : gr[v]) {
                if (par[p] == -1) {
                    par[p] = v;
                    st.push(p);
                }
            }
        } else {
            tpr.push_back(v);
            st.pop();
        }
    }
}

Vi sccResolve(const VVi& gr)
{
    int N = gr.size();
    Vi tpr;
    Vi par(N, -1);
    for (int i = 0; i < N; i++) {
        if (par[i] == -1) {
            tprsort(i, gr, tpr, par);
        }
    }
    Vi ret(N, -1);
    int now = 0;
    for (int i = N - 1; i >= 0; i--) {
        int u = tpr[i];
        if (ret[u] != -1) {
            continue;
        }
        ret[u] = now;
        stack<int> st;
        st.push(u);
        while (!st.empty()) {
            int v = st.top();
            st.pop();
            for (int p : gr[v]) {
                if (ret[p] == -1) {
                    st.push(p);
                    ret[p] = now;
                }
            }
        }
        now++;
    }
    return ret;
}

ll gcd(ll a, ll b)
{
    while (b) {
        a %= b;
        swap(a, b);
    }
    return a;
}
ll mpow(ll x, ll n, ll m)
{
    ll ret = 1;
    while (n) {
        if (n % 2) {
            ret *= x;
            ret %= m;
        }
        x = (x * x) % m;
        n /= 2;
    }
    return ret;
}
ll comb(int n, int r, const Vl& kai, const Vl& fkai, ll m)
{
    if (n < 0 || r < 0 || n < r) {
        return 0;
    }
    ll ret = kai[n];
    ret *= fkai[r];
    ret %= m;
    ret *= fkai[n - r];
    ret %= m;
    return ret;
}

int LCA(const VVi& par, const Vi& depth, int a, int b)
{
    if (depth[a] < depth[b]) {
        swap(a, b);
    }
    int dis = depth[a] - depth[b];
    for (int i = 19; i >= 0; i--) {
        if ((dis >> i) & 1) {
            a = par[i][a];
        }
    }
    if (a == b) {
        return a;
    }
    for (int i = 19; i >= 0; i--) {
        if (par[i][a] != par[i][b]) {
            a = par[i][a];
            b = par[i][b];
        }
    }
    return par[0][a];
}

void dijkstra(const VVi& gr, const VVl& cost, Vl& dist, int s)
{
    ll INF = (ll)1e18;
    dist.assign(gr.size(), INF);
    dist[s] = 0;
    PQmin<Pl> pque;
    pque.push(make_pair(0, s));
    while (!pque.empty()) {
        auto [L, u] = pque.top();
        pque.pop();
        while (L != dist[u] && !pque.empty()) {
            tie(L, u) = pque.top();
            pque.pop();
        }
        for (int i = 0; i < gr[u].size(); i++) {
            int v = gr[u][i];
            ll c = cost[u][i];
            if (dist[v] > c + L) {
                dist[v] = c + L;
                pque.push(make_pair(dist[v], v));
            }
        }
    }
    return;
}
unionfind::unionfind(int N)
{
    par.resize(N + 1);
    siz.assign(N + 1, 1);
    val.assign(N + 1, 0);
    for (int i = 0; i <= N; i++) {
        par[i] = i;
    }
}
unionfind::unionfind()
{
    par.resize(100010);
    siz.assign(100010, 1);
    val.assign(100010, 0);
    for (int i = 0; i <= 100010; i++) {
        par[i] = i;
    }
}
int unionfind::root(int v)
{
    if (v == par[v]) {
        return v;
    }
    return par[v] = root(par[v]);
}
void unionfind::merge(int a, int b)
{
    a = root(a);
    b = root(b);
    if (a == b) {
        return;
    }
    if (siz[a] < siz[b]) {
        int t = a;
        a = b;
        b = t;
    }
    par[b] = a;
    siz[a] += siz[b];
    /*
    ここにvalについての演算処理を書く
    */
}
bool unionfind::same(int a, int b)
{
    a = root(a);
    b = root(b);
    return a == b;
}
int unionfind::value(int a)
{
    return siz[this->root(a)];
}

BIT::BIT(int N)
{
    siz = N;
    bit.assign(N + 1, 0);
}

BIT::BIT()
{
    siz = 0;
}

ll& BIT::get(int id)
{
    return bit[id];
}

void BIT::add(int id, ll a, ll m)
{
    while (id <= siz) {
        bit[id] += a;
        bit[id] %= m;
        id += (id & (-id));
    }
}

ll BIT::sum(int id, ll m)
{
    ll ret = 0;
    while (id) {
        ret += bit[id];
        ret %= m;
        id -= (id & (-id));
    }
    return ret;
}