#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;

bool topological_sort(vector<vector<int>> g, vector<int> &order){
    int n = g.size();
    order.clear();
    vector<bool> used(n, false);
    function<void(int)> dfs = [&](int v){
        used[v] = true;
        for(int to : g[v]){
            if(!used[to]) dfs(to);
        }
        order.push_back(v);
    };
    for(int v = 0; v < n; v++){
        if(!used[v]) dfs(v);
    }
    reverse(order.begin(), order.end());
    vector<int> inv_order(n);
    for(int i = 0; i < n; i++) inv_order[order[i]] = i;
    for(int v = 0; v < n; v++){
        for(int u : g[v]){
            if(inv_order[v] > inv_order[u]) return false;
        }
    }
    return true;
}


int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    ll n, k; cin >> n >> k;
    vector<vector<int>> g(n);
    for(int i = 0; i < k; i++){
        int a, b; cin >> a >> b; a--; b--;
        if(a == b){
            cout << -1 << endl;
            return 0;
        }
        g[a].push_back(b);
    }
    vector<int> ord;
    if(topological_sort(g, ord)){
        vector<int> dp(n, 1);
        for(int v: ord){
            for(int to: g[v]) chmax(dp[to], dp[v]+1);
        }
        cout << *max_element(dp.begin(), dp.end()) << endl;
    }else{
        cout << -1 << endl;
    }
}