#include <bits/stdc++.h>

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define FOR(i,n,m) for(int i=(int)(n); i<=(int)(m); i++)
#define RFOR(i,n,m) for(int i=(int)(n); i>=(int)(m); i--)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define RITR(x,c) for(__typeof(c.rbegin()) x=c.rbegin();x!=c.rend();x++)
#define setp(n) fixed << setprecision(n)

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

#define ll long long
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll,ll>
#define pi pair<int,int>

#define all(a) (a.begin()),(a.end())
#define rall(a) (a.rbegin()),(a.rend())
#define fi first
#define se second
#define pb push_back
#define ins insert

#define debug(a) cerr<<(a)<<endl
#define dbrep(a,n) rep(_i,n) cerr<<(a[_i])<<" "; cerr<<endl
#define dbrep2(a,n,m) rep(_i,n){rep(_j,m) cerr<<(a[_i][_j])<<" "; cerr<<endl;}

using namespace std;

template<class A, class B>
ostream &operator<<(ostream &os, const pair<A,B> &p){return os<<"("<<p.fi<<","<<p.se<<")";}
template<class A, class B>
istream &operator>>(istream &is, pair<A,B> &p){return is>>p.fi>>p.se;}

template<class T>
vector<T> make_vec(size_t a){
	return vector<T>(a);
}
template<class T, class... Ts>
auto make_vec(size_t a, Ts... ts){
	return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

/* Some Libraries */

//-------------------------------------------------

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	int N,K; cin>>N>>K;

	vector<vi> G(N);
	rep(i,K){
		int a,b; cin>>a>>b; a--; b--;
		G[a].pb(b);
	}
	
	vi vec;

	bool ng = false;
	vector<bool> seen(N), finished(N);
	auto dfs = [&](auto &dfs, int v)->void{
		seen[v] = true;
		for(auto u:G[v]){
			if (seen[u] && !finished[u]){
				ng = true;
				continue;
			}
			if (seen[u]) continue;
			dfs(dfs,u);
		}
		finished[v] = true;
		vec.pb(v);
	};
	dfs(dfs,0);

	if (ng){
		cout<<"-1\n";
		return 0;
	}

	reverse(all(vec));

	vi dp(N,0);
	rep(i,N){
		for(auto u:G[vec[i]]){
			chmax(dp[u], dp[vec[i]]+1);
		}
	}
	int ans = *max_element(all(dp));
	cout<<ans+1<<"\n";
    return 0;
}