Educational DP Contest G - Longest Path

/*
 * @Descripttion:
 * @Author: Echo
 * @version:
 * @Date: 2022-08-29 17:04:52
 * @LastEditors: Echo
 * @LastEditTime: 2022-08-29 17:12:33
 */
#include <bits/stdc++.h>
#define rep(i, x, y) for (int i = x; i <= y; ++i)
#define per(i, x, y) for (int i = x; i >= y; --i)
#define mem(a, b) memset(a, b, sizeof a)
#define INF 0x3f3f3f3f
#define ll long long
#define pushk push_back
using namespace std;
const int N = 1e5 + 9;
vector<int> g[N];
int f[N];
bool st[N];
void dfs(int u, int fa){
   if(f[u]) return ;
   for(auto & it : g[u]){
        if(it!=fa){
            dfs(it, u);
            f[u] = max(f[u], f[it]+1);
        }
   }
}
int main()
{
    int n, m;
    cin >> n >> m;
    rep(i, 1, m)
    {
        int u,v;
        scanf("%d %d",&u,&v);
        g[u].pushk(v);
    }
    rep(i,1,n){
        if(!st[i]){
            st[i] = 1;
            f[0] = -1;
            dfs(i,0);
        }
    }
    int ans=0;
    rep(i,1,n) ans=max(ans,f[i]);
    cout<<ans;
    return 0;
}