#include<bits/stdc++.h>
using namespace std;

int main(){
    int64_t res = 0;
    int64_t N, K; cin >> N >> K;
    if(K == 0){
        cout << 1 << '\n';
        return 0;
    }
    vector<vector<int64_t>> edge(N);
    vector<int64_t> in_count(N, 0);
    while(K--){
        int64_t r, c; cin >> r >> c; r--; c--;
        edge[c].push_back(r);
        in_count[r]++;
    }
    queue<pair<int64_t,int64_t>> que;
    for(int64_t i = 0; i < N; i++) if(in_count[i] == 0) que.push({i, 0});
    while(not que.empty()){
        auto [vis, c] = que.front(); que.pop();
        res = max(res, c);
        for(auto nxt : edge[vis]) if(--in_count[nxt] == 0) que.push({nxt, c+1});
    }
    cout << (none_of(in_count.begin(), in_count.end(), [](auto x){return x > 0;}) ? res+1 : -1) << '\n';
}