/**
*    author:  tomo0608
*    created: 27.08.2021 23:35:26
**/

#ifdef __LOCAL
#define _GLIBCXX_DEBUG
#endif
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
#if __has_include(<atcoder/all>)
#include<atcoder/all>
using namespace atcoder;
#endif

typedef long long ll;
typedef long double ld;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
template <class T> using VVV = V<VV<T>>;
typedef pair<int,int> pii;
typedef pair<long long, long long> pll;

#define all(x) x.begin(),x.end()
#define SUM(v) accumulate(all(v), 0LL)
#define MIN(v) *min_element(all(v))
#define MAX(v) *max_element(all(v))
#define lb(c, x) distance((c).begin(), lower_bound(all(c), (x)))
#define ub(c, x) distance((c).begin(), upper_bound(all(c), (x)))
#define UNIQUE(x) sort(all(x)), x.erase(unique(all(x)), x.end())
template <typename T> void zip(vector<T> &x) {vector<T> y = x;UNIQUE(y);for(int i = 0; i < x.size(); ++i) { x[i] = lb(y, x[i]); }}
#define rep1(a)          for(long long i = 0; i < a; i++)
#define rep2(i, a)       for(long long i = 0; i < a; i++)
#define rep3(i, a, b)    for(long long i = a; i < b; i++)
#define rep4(i, a, b, c) for(long long i = a; i < b; i += c)
#define drep1(a)          for(long long i = a-1; i >= 0; i--)
#define drep2(i, a)       for(long long i = a-1; i >= 0; i--)
#define drep3(i, a, b)    for(long long i = a-1; i >= b; i--)
#define drep4(i, a, b, c) for(long long i = a-1; i >= b; i -= c)
#define overload4(a, b, c, d, e, ...) e
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)
#define drep(...) overload4(__VA_ARGS__, drep4, drep3, drep2, drep1)(__VA_ARGS__)
template<class... T>void input(T&... a){(cin >> ... >> a);};
#define INT(...) int __VA_ARGS__; input(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__; input(__VA_ARGS__)
#define STR(...) string __VA_ARGS__; input(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__; input(__VA_ARGS__)
void print(){cout << '\n';}
template<class T, class... Ts>void print(const T& a, const Ts&... b){cout << a; (cout << ... << (cout << ' ', b)); cout << '\n';}
void drop(){cout << '\n';exit(0);}
template<class T, class... Ts>void drop(const T& a, const Ts&... b){cout << a; (cout << ... << (cout << ' ', b)); cout << '\n';exit(0);}
template<class T> using priority_queue_rev = priority_queue<T, vector<T>, greater<T> >;
template<class T, class U> inline bool chmax(T &a, const U &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T, class U> inline bool chmin(T &a, const U &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T1, class T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }
template<class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { os << '(' << p.first << ", " << p.second << ')'; return os; }
template<class T> istream &operator>>(istream &is, vector<T> &v) { for (auto &e : v) is >> e; return is; }
template<class T> ostream &operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << ' '; return os; }
template<typename T> ostream& operator << (ostream& os, set<T>& set_var) {os << "{"; for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {os << *itr;++itr;if(itr != set_var.end()) os << ", ";itr--;}os << "}";return os;}
template <typename T, typename U> ostream &operator<<(ostream &os, map<T, U> &map_var) {os << "{";for(auto itr = map_var.begin(); itr != map_var.end(); itr++) {os << *itr;itr++;if (itr != map_var.end()) os << ", ";itr--;}os << "}";return os;}
#ifdef __LOCAL
void debug_out(){ cerr << endl;}
template < class Head, class... Tail> void debug_out(Head H, Tail... T) { cerr << ' ' << H; debug_out(T...);}
#define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
template<class T> inline int count_between(vector<T> &a, T l, T r) { return lower_bound(all(a), r) - lower_bound(all(a), l); } // [l, r)
#define mp make_pair
#define bit(n, k) ((n >> k) & 1) /*nのk bit目*/
int topbit(signed t) { return t == 0 ? -1 : 31 - __builtin_clz(t); }
int topbit(ll t) { return t == 0 ? -1 : 63 - __builtin_clzll(t); }
int lowbit(signed a) { return a == 0 ? 32 : __builtin_ctz(a); }
int lowbit(ll a) { return a == 0 ? 64 : __builtin_ctzll(a); }
template <typename T>T mypow(T x, ll n){T ret = 1;while(n > 0){if(n & 1)(ret *= x);(x *= x);n >>= 1;}return ret;}
#define endl '\n'
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, -1, 1};

bool visit(vector<vector<int> > &graph, vector<int> & dag, vector<bool> & use, vector<bool> & finished, int s){
    use[s] = false;
    finished[s] = false;
    bool c = true;
    for(int to: graph[s]){
        if(!finished[to])return false;
        if(use[to])c = c && visit(graph, dag, use, finished, to);
    }
    finished[s] = true;
    dag.push_back(s);
    return c;
}
bool topological_sort(vector<vector<int> > &graph, vector<int> & dag){// graph をもとにトポロジカルソート後の数をdagに, 閉路がある場合はfalse
    int n = graph.size();
    bool c = true;
    vector<bool> use(n, true), finished(n, true);
    for(int i = 0; i < n; i++){
        if(use[i])c = c && visit(graph, dag, use, finished, i);
    }
    reverse(all(dag));
    return c;
}

void solve(){
    INT(n,k);
    VV<int> g(n);
    bool check = false;
    rep(i,k){
        INT(r,c);
        r--,c--;
        g[r].emplace_back(c);
    }
    V<int> dag;
    if(!topological_sort(g, dag)){
        drop(-1);
    }
    reverse(all(dag));
    V<int> dist(n, -1);
    auto dfs = [&](int now)->ll{
        if(dist[now] >= 0)return dist[now];
        dist[now] = 1;
        for(int to: g[now])chmax(dist[now], dist[to]+1);
        return dist[now];
    };
    ll m = 0;
    for(int i: dag)chmax(m, dfs(i));
    debug(dist);
    print(m);
}

int main() {
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    cout << setprecision(20);
    int codeforces = 1;
    //cin >> codeforces;
    while(codeforces--){
        solve();
    }
    return 0;
}