import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.range, std.regex, std.typecons;
import core.bitop, core.thread;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) throw new EOFException; tokens = readln.split; } auto token = tokens[0]; tokens.popFront; return token; }
int readInt() { return readToken().to!int; }
long readLong() { return readToken().to!long; }
real readReal() { return readToken().to!real; }

void chmin(T)(ref T t, in T f) { if (t > f) t = f; }
void chmax(T)(ref T t, in T f) { if (t < f) t = f; }

int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = cast(int)(as.length); for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a < val)); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a <= val)); }


int N, M;
int[] X, Y;

int[][] G;
int[] dp;

void solve(int u) {
  if (dp[u] == -1) {
    foreach (v; G[u]) {
      solve(v);
    }
    dp[u] = 0;
    foreach (v; G[u]) {
      chmax(dp[u], 1 + dp[v]);
    }
  }
}

void main() {
  try {
    for (; ; ) {
      N = readInt();
      M = readInt();
      X = new int[M];
      Y = new int[M];
      foreach (i; 0 .. M) {
        X[i] = readInt() - 1;
        Y[i] = readInt() - 1;
      }
      G = new int[][N];
      foreach (i; 0 .. M) {
        G[X[i]] ~= Y[i];
      }
      dp = new int[N];
      dp[] = -1;
      foreach (u; 0 .. N) {
        solve(u);
      }
      int ans;
      foreach (u; 0 .. N) {
        chmax(ans, dp[u]);
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}
