#include <bits/stdc++.h>
#include <boost/multiprecision/cpp_int.hpp>
using namespace std;
namespace mp = boost::multiprecision;

int main(){
    int N,K;
    cin>>N>>K;
    vector<vector<int>> edges(K+N,vector<int>(2));
    vector<int> used(N);
    for(int i=0;i<K;++i){
        int r,c;
        cin>>r>>c;
        used[c-1]=1;
        edges[i]={r-1,c-1};
    }
    int pointer=K;
    for(int i=0;i<N;i++){
        if(!used[i]){
            edges[pointer]={N,i};
            ++pointer;
        }
    }
    edges.resize(pointer);
    vector<int> value(N+1,-1);
    vector<int> before(N+1);
    vector<vector<int>> front(N+1,vector<int>(0));
    vector<vector<int>> parent(N+1,vector<int>(0));
    for(vector<int> i:edges){
        ++before[i[1]];
        front[i[0]].push_back(i[1]);
        parent[i[1]].push_back(i[0]);
    }
    queue<int> q;
    q.push(N);
    while(!q.empty()){
        int p=-1;
        for(int i:parent[q.front()]){
            p=max(p,value[i]);
        }
        value[q.front()]=p+1;
        for(int i:front[q.front()]){
            --before[i];
            if(!before[i]){
                q.push(i);
            }
        }
        q.pop();
    }
    int ans=0;
    for(int i=0;i<N;i++){
        if(value[i]==-1){
            cout<<-1<<endl;
            return 0;
        }
        ans=max(ans,value[i]);
    }
    cout<<ans<<endl;
}