#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=2e9+1;
const ll INF=4e18;
const ll dy[8]={-1,0,1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template <typename T> inline bool chmax(T &a, T b) {
  return ((a < b) ? (a = b, true) : (false));
}
template <typename T> inline bool chmin(T &a, T b) {
  return ((a > b) ? (a = b, true) : (false));
}

vl topsort(vvl &g){
    ll n=g.size();
    vl ans,ind(n);
    rep(i,n)for(auto e:g[i])ind[e]++;
    queue<ll> que;
    rep(i,n)if(!ind[i])que.push(i);
    while(!que.empty()){
        ll v=que.front();que.pop();
        ans.pb(v);
        for(auto nv:g[v]){
            ind[nv]--;if(!ind[nv])que.push(nv);
        }
    }
    return ans;
}

int main(){
  ll n,m;cin >> n >> m;
  vvl g(n);
  rep(i,m){
    ll a,b;cin >> a >> b;a--;b--;
    g[a].push_back(b);
  }
  auto ts=topsort(g);
  if(ts.size()!=n)cout << -1 << endl,exit(0);
  vl dp(n);
  for(auto p:ts){
    for(auto to:g[p]){
      chmax(dp[to],dp[p]+1);
    }
  }
  cout << *max_element(all(dp))+1 << endl;
}