#include <bits/stdc++.h>
#define ll long long
#define mod 1000000007
#define pii pair<int, int>
#define fr(n) for (int i = 0; i < n; i++)
#define fr1(n) for (int i = 1; i <= n; i++)
using namespace std;

void A() {
    int n, a[100005] = {}, b[100005] = {};
    cin >> n;
    fr1(n) cin >> a[i];
    b[1] = 0; b[2] = abs(a[1] - a[2]);
    for (int i = 3; i <= n; i++) b[i] = min(b[i - 2] + abs(a[i - 2] - a[i]), b[i - 1] + abs(a[i - 1] - a[i]));
    cout << b[n];
}

void B() {
    int n, k, a[100005] = {}, b[100005] = {};
    cin >> n >> k;
    fr1(n) cin >> a[i], b[i] = mod;
    fr1(k) b[i] = abs(a[1] - a[i]);
    for (int i = k + 1; i <= n; i++) for (int j = 1; j <= k; j++) b[i] = min(b[i - j] + abs(a[i - j] - a[i]), b[i]);
    cout << b[n];
}

void C() {
    int n, a[100005][3] = {}, b[100005][3] = {};
    cin >> n;
    fr1(n) cin >> a[i][0] >> a[i][1] >> a[i][2];
    fr1(n) b[i][0] = max(b[i - 1][1], b[i - 1][2]) + a[i][0], b[i][1] = max(b[i - 1][2], b[i - 1][0]) + a[i][1], b[i][2] = max(b[i - 1][0], b[i - 1][1]) + a[i][2];
    cout << max({b[n][0], b[n][1], b[n][2]});
}

void D() {
    int n, w, a[102] = {}, b[102] = {};
    ll z = 0, d[100005] = {};
    cin >> n >> w;
    fr(n) cin >> a[i] >> b[i];
    fr(n) for (int j = w; j >= a[i]; j--) d[j] = max(d[j], d[j - a[i]] + b[i]);
    fr1(w) z = max(z, d[i]);
    cout << z;
}

void E() {
    int n, a[102] = {}, b[102] = {};
    ll w, d[100005] = {};
    cin >> n >> w;
    fr1(100000) d[i] = mod * 328ll;
    fr(n) cin >> a[i] >> b[i];
    fr(n) for (int j = 100000; j >= b[i]; j--) d[j] = min(d[j], d[j - b[i]] + a[i]);
    for (int i = 100000; i >= 0; i--) if (d[i] <= w) {
        cout << i;
        break;
    }
}

void F() {
    string s, t, u;
    cin >> s >> t;
    int d[3003][3003] = {}, n = (int)s.length(), m = (int)t.length();
    fr1(n) for (int j = 1; j <= m; j++) {
        if (s[i - 1] == t[j - 1]) d[i][j] = d[i - 1][j - 1] + 1;
        else d[i][j] = max(d[i - 1][j], d[i][j - 1]);
    }
    while (n && m) {
        if (d[n - 1][m - 1] + 1 == d[n][m]) n--, m--, u += s[n];
        else if (n && d[n - 1][m] == d[n][m]) n--;
        else m--;
    }
    reverse(u.begin(), u.end());
    cout << u;
}

void G() {
    int n, m, u, v, z = 0, c[100005] = {}, d[100005] = {};
    vector<int> g[100005] = {};
    cin >> n >> m;
    fr(m) cin >> u >> v, g[u].push_back(v), c[v]++;
    queue<int> q;
    fr1(n) if (!c[i]) q.push(i);
    while (!q.empty()) {
        int x = q.front();
        q.pop();
        for (int i : g[x]) {
            d[i] = max(d[i], d[x] + 1);
            c[i]--;
            if (!c[i]) q.push(i);
        }
    }
    fr1(n) z = max(z, d[i]);
    cout << z;
}

void H() {
    int n, m, d[1003][1003] = {1};
    string s[1003] = {};
    cin >> n >> m;
    fr(n) cin >> s[i];
    fr(n) for (int j = 0; j < m; j++) if (s[i][j] == '.') {
        if (i) d[i][j] += d[i - 1][j];
        if (j) d[i][j] += d[i][j - 1];
        if (d[i][j] >= mod) d[i][j] -= mod;
    }
    cout << d[n - 1][m - 1];
}

void I() {
    int n;
    double z = 0, a[3003] = {}, d[3003] = {1};
    cin >> n;
    fr1(n) cin >> a[i];
    fr1(n) for (int j = n; j >= 0; j--) {
        double e = d[j];
        if (j) d[j] = d[j - 1] * a[i];
        else d[j] = 0;
        d[j] += e * (1 - a[i]);
    }
    fr(n + 1) if (i + i > n) z += d[i];
    cout << setprecision(13) << z;
}

void J() {
    int n, c0, c[4] = {};
    double d[303][303][303] = {};
    cin >> n;
    fr(n + 1) for (int j = 0; j <= i; j++) for (int k = 0; k <= j; k++) {
        if (i - j) d[i][j][k] += d[i - 1][j][k] * (i - j) / i;
        if (j - k) d[i][j][k] += d[i][j - 1][k] * (j - k) / i;
        if (k) d[i][j][k] += d[i][j][k - 1] * k / i;
        if (i) d[i][j][k] += 1.0 * n / i;
        //cout << i << ' ' << j << ' ' << k << ' ' << d[i][j][k] << '\n';
    }
    fr(n) cin >> c0, c[c0]++;
    cout << setprecision(13) << d[c[1] + c[2] + c[3]][c[2] + c[3]][c[3]];
}

void K() {
    int n, k, a[102] = {}, b[100005] = {};
    cin >> n >> k;
    fr(n) cin >> a[i];
    fr1(k) for (int j = 0; j < n; j++) if (i >= a[j]) b[i] |= !b[i - a[j]];
    cout << (b[k] ? "First" : "Second");
}

void L() {
    int n;
    ll a[3003] = {}, d[2][3003][3003] = {};
    cin >> n;
    fr(n) cin >> a[i];
    fr(n) for (int j = 0; j + i < n; j++) {
        d[0][j][j + i] = max(d[1][j + 1][j + i] + a[j], d[1][j][j + i - 1] + a[j + i]);
        d[1][j][j + i] = min(d[0][j + 1][j + i], d[0][j][j + i - 1]);
    }
    cout << d[0][0][n - 1] - d[1][0][n - 1];
}

void M() {
    int n, k, a[102] = {};
    ll d[100005] = {1};
    cin >> n >> k;
    fr(n) cin >> a[i];
    for (int j = 0; j < n; j++) {
        ll e[100005] = {};
        fr(k + 1) e[i] = d[i];
        fr1(k) e[i] += e[i - 1];
        fr(k + 1) d[i] = e[i] - (i > a[j] ? e[i - a[j] - 1] : 0);
        fr(k + 1) d[i] %= mod;
    }
    cout << d[k];
}

void N() {
    int n;
    ll a[403] = {}, b[403] = {}, d[403][403] = {};
    cin >> n;
    fr1(n) cin >> a[i], b[i] = b[i - 1] + a[i];
    fr(403) for (int j = 0; j < 403; j++) d[i][j] = mod * 890328ll;
    fr1(n) d[i][i] = 0;
    for (int i = 1; i < n; i++) for (int j = 1; j + i <= n; j++) {
        for (int k = j; k < j + i; k++) d[j][j + i] = min(d[j][j + i], d[j][k] + d[k + 1][j + i]);
        d[j][j + i] += b[j + i] - b[j - 1];
    }
    cout << d[1][n] << '\n';
}

void O() {
    int n, d[2100328] = {1}, c[2100328] = {};
    vector<int> g[2100328] = {};
    bool b[22][22] = {};
    cin >> n;
    fr(1 << n) for (int j = 0; j < n; j++) if (i & (1 << j)) g[i].push_back(j), c[i]++;
    fr(n) for (int j = 0; j < n; j++) cin >> b[i][j];
    fr(n) {
        for (int j = (int)(1 << n) - 1; j >= 0; j--) if (c[j] == i + 1) {
            for (int k : g[j]) if (b[i][k]) {
                d[j] += d[j - (1 << k)];
                if (d[j] >= mod) d[j] -= mod;
            }
        }
    }
    cout << d[(1 << n) - 1];
}

vector<int> g[100005];
ll d[100005], e[100005];

void dfs(int x, int p) {
    d[x] = e[x] = 1;
    for (int i : g[x]) if (i != p) {
        dfs(i, x);
        d[x] = d[x] * e[i] % mod;
        e[x] = e[x] * (d[i] + e[i]) % mod;
    }
}

void P() {
    int n, u, v;
    cin >> n;
    fr(n - 1) cin >> u >> v, g[u].push_back(v), g[v].push_back(u);
    dfs(1, 1);
    cout << (d[1] + e[1]) % mod;
}

void Q() {
    int n, h[200005] = {};
    ll t[200005] = {}, a[200005] = {}, y = 0;
    cin >> n;
    fr1(n) cin >> h[i];
    fr1(n) cin >> a[i];
    fr1(n) {
        ll z = 0;
        for (int j = h[i]; j; j -= j & -j) z = max(z, t[j]);
        for (int j = h[i]; j <= n; j += j & -j) t[j] = max(t[j], z + a[i]);
    }
    fr1(n) y = max(y, t[i]);
    cout << y;
}

vector<vector<ll>> mul(int n, vector<vector<ll>> c, vector<vector<ll>> d) {
    vector<vector<ll>> e(n);
    fr(n) e[i].resize(n);
    fr(n) for (int j = 0; j < n; j++) {
        for (int k = 0; k < n; k++) e[i][j] = (e[i][j] + c[i][k] * d[k][j]) % mod;
    }
    return e;
}

void R() {
    int n;
    vector<vector<ll>> a, b;
    ll m, z = 0;
    cin >> n >> m;
    a.resize(n); b.resize(n);
    fr(n) a[i].resize(n), b[i].resize(n);
    fr(n) for (int j = 0; j < n; j++) cin >> a[i][j];
    fr(n) b[i][i] = 1;
    while (m) {
        if (m & 1) b = mul(n, b, a);
        a = mul(n, a, a);
        m >>= 1;
    }
    fr(n) for (int j = 0; j < n; j++) z += b[i][j];
    cout << z % mod;
}

void S() {
    string k;
    int m;
    ll d[10004][102][2] = {};
    d[0][0][1] = 1;
    cin >> k >> m;
    fr1(k.length()) {
        for (int j = 0; j < m; j++) {
            for (int l = 0; l < 10; l++) d[i][(j + l) % m][0] += d[i - 1][j][0];
            for (int l = 0; l < k[i - 1] - '0'; l++) d[i][(j + l) % m][0] += d[i - 1][j][1];
            d[i][(j + k[i - 1] - '0') % m][1] = d[i - 1][j][1];
        }
        for (int j = 0; j < m; j++) d[i][j][0] %= mod;
    }
    cout << (d[k.length()][0][0] + d[k.length()][0][1] + mod - 1) % mod;
}

void U() {
    int n;
    ll a[17][17] = {}, d[65541] = {}, s[65541] = {}, z = 0;
    cin >> n;
    fr(n) for (int j = 0; j < n; j++) cin >> a[i][j];
    fr(1 << n) for (int j = 0; j < n; j++) for (int k = 0; k < j; k++) if (i & (1 << j) && i & (1 << k)) s[i] += a[j][k];
    fr(1 << n) {
        vector<int> b;
        for (int j = 0; j < n; j++) if (i & (1 << j)) b.push_back(1 << j);
        int m = (int)b.size();
        for (int j = 0; j < (1 << m); j++) {
            int c = 0;
            for (int k = 0; k < m; k++) if (j & (1 << k)) c += b[k];
            d[i] = max(d[i], d[i - c] + s[c]);
        }
    }
    fr(1 << n) z = max(z, d[i]);
    cout << z;
}

//vector<int> g[100005];
int p[100005];
ll d2[100005] = {1}, z[100005], m;

void dfs2(int x, int w) {
    d2[x] = 1;
    for (int i : g[x]) if (i != w) p[i] = x, dfs2(i, x), d2[x] = d2[x] * (d2[i] + 1) % m;
    if (x > 1) g[x].erase(find(g[x].begin(), g[x].end(), w));
}

void dfs3(int x) {
    vector<ll> a = {1}, b = {1};
    for (int i = (int)g[x].size() - 1; i >= 0; i--) a.push_back(a.back() * (d2[g[x][i]] + 1) % m);
    fr(g[x].size()) b.push_back(b.back() * (d2[g[x][i]] + 1) % m);
    reverse(a.begin(), a.end());
    z[x] = d2[x];
    fr(g[x].size()) {
        d2[x] = d2[p[x]] * (b[i] * a[i + 1] % m) % m + 1;
        d2[g[x][i]] = d2[g[x][i]] * d2[x] % m;
    }
    for (int i = 0; i < g[x].size(); i++) d2[x] = d2[p[x]] * (b[i] * a[i + 1] % m) % m + 1, dfs3(g[x][i]);
}

void V() {
    int n, u, v;
    cin >> n >> m;
    fr1(n - 1) cin >> u >> v, g[u].push_back(v), g[v].push_back(u);
    dfs2(1, 1);
    dfs3(1);
    fr1(n) cout << z[i] << '\n';
}

void X() {
    int n;
    ll z = 0, d[20004] = {};
    pair<pii, int> a[1003] = {};
    cin >> n;
    fr(n) cin >> a[i].first.second >> a[i].first.first >> a[i].second, a[i].first.first += a[i].first.second;
    sort(a, a + n);
    fr(n) for (int j = a[i].first.first; j >= a[i].first.second; j--) d[j] = max(d[j], d[j - a[i].first.second] + a[i].second);
    fr(20003) z = max(z, d[i]);
    cout << z;
}

void Y() {
    ll f[200005] = {1}, g[200005] = {}; g[200000] = 750007460;
    fr1(200000) f[i] = f[i - 1] * i % mod;
    for (int i = 200000; i; i--) g[i - 1] = g[i] * i % mod;
    int h, w, n;
    ll d[3003] = {};
    pii a[3003] = {};
    cin >> h >> w >> n;
    a[n].first = h, a[n].second = w;
    fr(n) cin >> a[i].first >> a[i].second;
    sort(a, a + n);
    fr(n + 1) d[i] = f[a[i].first + a[i].second - 2] * g[a[i].first - 1] % mod * g[a[i].second - 1] % mod;
    fr(n + 1) {
        for (int j = 0; j < i; j++) if (a[i].first >= a[j].first && a[i].second >= a[j].second) {
            d[i] -= d[j] * f[a[i].first - a[j].first + a[i].second - a[j].second] % mod * g[a[i].first - a[j].first] % mod * g[a[i].second - a[j].second] % mod;
            if (d[i] < 0) d[i] += mod;
        }
    }
    cout << d[n];
}

int main() {
    ios::sync_with_stdio(false);
    G();
}


Measured memory_bytes: 10368