#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;

vector<pair<int, int>> mult(const vector<vector<int>> &f, const vector<pair<int, int>> &g) {
    vector<pair<int, int>> h;
    for (auto [z, x] : g) {
        for (int y : f[z]) {
            h.emplace_back(y, x);
        }
    }
    sort(ALL(h));
    h.erase(unique(ALL(h)), h.end());
    return h;
}

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

    int ans = 1;
    while (not g.empty()) {
        for (auto [y, x] : g) {
            if (y == x) {
                return -1;
            }
        }
        g = mult(f, g);
        ans += 1;
    }
    return ans;
}

// 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;
}