G.cpp

#include <bits/stdc++.h>
#define ll long long
#define dbg(x)  cout<<#x<<" "<<x<<endl;
#define F first
#define S second
#define setbit(n) __builtin_popcount(n)
#define all(x) x.begin() , x.end()
#define clr(x) memset(x,0,sizeof(x))
#define pb push_back
#define mp make_pair
#define ub upper_bound
#define lb lower_bound
#define endl "\n" //delete if interactive
using namespace std;

void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i : x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ASHISH_DANGI
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif

const int N =  1e5 + 1 ;
vector<int>graph[N];
vector<bool>visited(N , false) ;
vector<ll> cost(N , 0 ) ;


void dfs(int sv)
{
	visited[sv] = 1;

	ll ans = 0 ;
	for (int child :  graph[sv])
	{
		if (!visited[child])
			dfs(child);

		ans = max(ans , cost[child]);

	}

	cost[sv] = ans + 1 ;
}
void solve()
{
	int n , m  , x, y;
	cin >> n >> m ;

	for (int i = 0 ; i < m ; i++)
	{
		cin >> x >> y ;
		graph[x].push_back(y);
	}

	for (int i = 1; i <= n ; i++)
		if (!visited[i])
			dfs(i);
	ll ans  = 0 ;
	for (int i = 1 ; i <= n ; i++)
		ans = max(ans , cost[i]);

	cout << ans - 1 << endl;


}
int main() {
	// your code goes here
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);


	solve();

	return 0;
}