g.cpp

#include "bits/stdc++.h"
using namespace std;
#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
  enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "

using ll = long long;

// DAG - directed acyclic graph

const int nax = 1e5 + 5;
vector<int> edges[nax];
int in_degree[nax]; // the number of edges going to 'b'
int dist[nax];
bool visited[nax];

void dfs(int a) {
    assert(!visited[a]);
    visited[a] = true;
    for(int b : edges[a]) {
        dist[b] = max(dist[b], dist[a] + 1);
        --in_degree[b];
        if(in_degree[b] == 0) {
            dfs(b);
        }
    }
}

int main() {
    int n, m;
    scanf("%d%d", &n, &m);
    while(m--) {
        int a, b;
        scanf("%d%d", &a, &b);
        edges[a].push_back(b);
        ++in_degree[b];
    }
    for(int i = 1; i <= n; ++i) {
        if(!visited[i] && in_degree[i] == 0) {
            dfs(i);
        }
    }
    int answer = 0;
    for(int i = 1; i <= n; ++i) {
        answer = max(answer, dist[i]);
    }
    printf("%d\n", answer);
}