#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace std;
using namespace atcoder;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define Rreps(i,n,e) for(int i = n - 1; i >= e; --i)
#define Rrep(i,n) Rreps(i,n,0)
#define ALL(a) a.begin(), a.end()

using ll = long long;
using vec = vector<ll>;
using mat = vector<vec>;

ll N,M,H,W,Q,K,A,B;
string S;
using P = pair<ll, ll>;
using tp = tuple<ll, ll, ll>;
const ll INF = (1LL<<60);

template<class T> bool chmin(T &a, const T b){
    if(a > b) {a = b; return true;}
    else return false;
}
template<class T> bool chmax(T &a, const T b){
    if(a < b) {a = b; return true;}
    else return false;
}
template<class T> void my_printv(std::vector<T> v,bool endline = true){
    if(!v.empty()){
        for(std::size_t i{}; i<v.size()-1; ++i) std::cout<<v[i]<<" ";
        std::cout<<v.back();
    }
    if(endline) std::cout<<std::endl;
}

using mint = modint1000000007 ;
using vm = vector<mint>;

void no(){
    cout<<-1<<endl;
    exit(0);
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cin>>N>>K;
    mat G(N + 1);
    rep(i, K){
        int r, c;
        cin>>r>>c;
        --r; --c;
        if(r == c) no();
        G[r].push_back(c);
    }
    {
        vec edge_num(N, 0);
        for(vec &v : G) for(int to : v) ++edge_num[to];
        rep(i, N) if(edge_num[i] == 0) G[N].push_back(i);
    }
    queue<int> que;
    vec dp(N + 1, 0);
    que.push(N);
    while(!que.empty()){
        int v = que.front();
        que.pop();
        for(int to : G[v]){
            if(chmax(dp[to], dp[v] + 1)){
                que.push(to);
            }
        }
    }
    cout<<(*max_element(ALL(dp)))<<endl;
}