#include <algorithm>
#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <iostream>
#include <numeric>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <functional>
#include <iomanip>
using namespace std;
using ll = long long;

class range {private: struct I{int x;int operator*(){return x;}bool operator!=(I& lhs){return x<lhs.x;}void operator++(){++x;}};I i,n;
public:range(int n_):i({0}),n({n_}){}range(int i_,int n_):i({i_}),n({n_}){}I& begin(){return i;}I& end(){return n;}};

template<typename T, typename U> ostream& operator<<(ostream& os, const pair<T, U>& p){ return os << "{" << p.first << ", " << p.second << "}"; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& obj) { os << "{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T> ostream& operator<<(ostream& os, const set<T>& obj) { os << "set{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T, typename U> ostream& operator<<(ostream& os, const map<T, U>& obj) { os << "map{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }

#ifdef ONLINE_JUDGE
#define dump(expr) ;
#else
#define dump(expr) { cerr << "\033[33m#L" << __LINE__ << ": " << expr << "\033[39m" << endl; }
#endif

ll solve() {
  int n, m; cin >> n >> m;
  vector<vector<int>> vg(n + 1);
  for (int i : range(m)) {
    int a, b; cin >> a >> b;
    vg[a].push_back(b);
  }

  if (m == 0) return 1;

  vector<int> ans(n + 1);
  auto dfs = [&](auto self, int idx) -> int {
    if (ans[idx]) return ans[idx];
    ans[idx] = n + 1;

    int res = 1;
    for (int to : vg[idx]) {
      res = max(res, 1 + self(self, to));
    }
    res = min(res, n + 1);
    return ans[idx] = res;
  };

  int res = 1;
  for (int i : range(n + 1)) res = max(res, dfs(dfs, i));

  for (int i : range(n + 1)) dump(i << " " << dfs(dfs, i));

  return (res >= n + 1) ? -1 : res;
}

int main() {
  cout << fixed << setprecision(12);
  cout << solve() << endl;
}