#include <bits/stdc++.h>

using namespace std;
void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}

int main() {
    fast_io();
    int n, k;
    cin >> n >> k;
    vector<vector<int>> g(n);
    vector<int> in_deg(n);
    for (int i = 0; i < k; i++) {
        int r, c;
        cin >> r >> c;
        g[r - 1].push_back(c - 1);
        in_deg[c - 1]++;
    }
    vector<int> top;
    deque<int> q;
    for (int i = 0; i < n; i++) {
        if (in_deg[i] == 0) {
            q.push_back(i);
        }
    }
    while (!q.empty()) {
        int v = q.front();
        q.pop_front();
        top.push_back(v);
        for (int u : g[v]) {
            in_deg[u]--;
            if (in_deg[u] == 0) {
                q.push_back(u);
            }
        }
    }
    if (top.size() < n) {
        cout << -1 << endl;
        return 0;
    }
    vector<int> dp(n);
    for (int v : top) {
        for (int u : g[v]) {
            dp[u] = max(dp[u], dp[v] + 1);
        }
    }
    cout << *max_element(dp.begin(), dp.end()) + 1 << endl;
}