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

#define dbg(x...) \
do { \
std::cout << #x << " -> "; \
err(x); \
} while (0)

void err() {
	std::cout << std::endl;
}

template<class T, class... Ts>
void err(T arg, Ts &... args) {
	std::cout << fixed << setprecision(10) << arg << ' ';
	err(args...);
}

void GENSHEN_START() {
	int n,m;cin >> n >> m;
	vector <int> du(n + 5);
	vector <vector<int>> g(n + 5);
	for (int i = 1;i <= m;i++) {
		int u,v;cin >> u >> v;
		g[u].push_back(v);
		du[v]++;
	}
	vector <int> dis(n + 5,-1);
	dis[1] = 0;
	queue <int> q;
	for (int i = 1;i <= n;i++) {
		if (du[i] == 0 && i != 1) {
			q.push(i);
		}
	}
	while (!q.empty()) {
		auto now = q.front();q.pop();
		for (auto i : g[now]) {
			du[i]--;
			if (du[i] == 0) q.push(i);
		}
	}
	q.push(1);
	while (!q.empty()) {
		auto now = q.front();q.pop();
		for (auto i : g[now]) {
			du[i]--;
			dis[i] = max(dis[i],dis[now] + 1);
			if (du[i] == 0) q.push(i);
		}
	}
	cout << max((int)0,dis[n]);
}

signed main()
{
	ios::sync_with_stdio(false);cin.tie(nullptr);
	int T = 1;
	//cin >> T;
	while (T--) GENSHEN_START();
	return 0;
}