import std.stdio, std.string, std.conv, std.range;
import std.algorithm, std.array, std.typecons, std.container;
import std.math, std.numeric, std.random, core.bitop;

enum inf = 1_001_001_001;
enum infl = 1_001_001_001_001_001_001L;
enum mod = 1_000_000_007L;


void main() {
    int N, M;
    scan(N, M);

    auto adj = new int[][](N, 0);
    foreach (i ; 0 .. M) {
        int xi, yi;
        scan(xi, yi);
        xi--, yi--;
        adj[xi] ~= yi;
    }

    auto ord = topSort(adj);
    auto dp = new int[](N);

    int ans;

    foreach (v ; ord) {
        ans = max(ans, dp[v]);
        foreach (u ; adj[v]) {
            chmax(dp[u], dp[v] + 1);
        }
    }

    writeln(ans);
}

int[] topSort(int[][] adj) {
    auto N = adj.length.to!int;
    auto deg = new int[](N);
    foreach (v ; 0 .. N) {
        foreach (u ; adj[v]) {
            deg[u]++;
        }
    }
    int[] res;
    int[] next;
    foreach (v ; 0 .. N) {
        if (!deg[v]) next ~= v;
    }

    while (!next.empty) {
        int[] nn;

        foreach (v ; next) {
            res ~= v;
            foreach (u ; adj[v]) {
                deg[u]--;
                if (!deg[u]) nn ~= u;
            }
        }

        next = nn;
    }

    return res;
}



void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}

void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}

bool chmin(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x > arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}

bool chmax(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x < arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}
