#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define ALL(x) std::begin(x), std::end(x)
using namespace std;

int64_t solve(int n, int k, const std::vector<int> &r, const vector<int> &c) {
    vector<vector<int>> g(n);
    REP (i, k) {
        g[r[i]].push_back(c[i]);
    }

    vector<int> dp(n, -1);
    vector<bool> stk(n);
    auto go = [&](auto &&go, int y) -> bool {
        stk[y] = true;
        dp[y] = 1;
        for (int x : g[y]) {
            if (stk[x]) {
                return false;
            }
            if (dp[x] == -1) {
                if (not go(go, x)) {
                    return false;
                }
            }
            dp[y] = max(dp[y], dp[x] + 1);
        }
        stk[y] = false;
        return true;
    };
    REP (y, n) {
        if (dp[y] == -1) {
            if (not go(go, y)) {
                return -1;
            }
        }
    }
    return *max_element(ALL(dp));
}

// generated by oj-template v4.8.0 (https://github.com/online-judge-tools/template-generator)
int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int N, K;
    std::cin >> N >> K;
    std::vector<int> r(K), c(K);
    REP (i, K) { std::cin >> r[i] >> c[i]; -- r[i]; -- c[i]; }
    auto ans = solve(N, K, r, c);
    std::cout << ans << '\n';
    return 0;
}