#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
#define rep(i,n) for(int i=0; i<(n); i++)


#include <vector>

vector<vector<int>> SCC(const vector<vector<int>>& E){
  int n = E.size();
  vector<int> F(n, 0);
  vector<int> St;
  vector<int> I;
  for(int i=0; i<n; i++) if(F[i] == 0){
    St.push_back(i);
    while(St.size()){
      int p = St.back();
      St.pop_back();
      if(p < 0){
        I.push_back(-1-p);
        continue;
      }
      if(F[p] != 0) continue;
      F[p] = 1;
      St.push_back(-1-p);
      for(int e : E[p]) St.push_back(e);
    }
  }
  vector<vector<int>> R(n);
  vector<vector<int>> G;
  for(int i=0; i<n; i++) for(int e : E[i]){
    R[e].push_back(i);
  }
  for(int i=n-1; i>=0; i--){
    int s = I[i];
    if(F[s] == 0) continue;
    vector<int> g;
    St.push_back(s);
    while(St.size()){
      int p = St.back();
      St.pop_back();
      if(F[p] == 0) continue;
      F[p] = 0;
      g.push_back(p);
      for(int e : R[p]) St.push_back(e);
    }
    G.emplace_back(move(g));
  }
  for(int i=0; i<G.size(); i++){
    for(int p : G[i]) F[p] = i;
  }
  return move(G);
}



int main(){
  int N; cin >> N;
  int K; cin >> K;
  if(K == 0){ cout << "0\n"; return 0; }
  vector<vector<int>> E(N);
  rep(i,K){
    int u,v; cin >> u >> v; u--; v--;
    if(u == v){ cout << "-1\n"; return 0; }
    E[u].push_back(v);
  }
  auto G = SCC(E);
  if(G.size() != N){ cout << "-1\n"; return 0; }
  vector<int> D(N,1);
  int ans = 1;
  for(auto& A : G) for(auto p : A) for(int e : E[p]) D[e] = max(D[e], D[p]+1);
  rep(i,N) ans = max(ans,D[i]);
  cout << ans << endl;
  return 0;
}

struct ios_do_not_sync{
  ios_do_not_sync(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
  }
} ios_do_not_sync_instance;