#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<int> mult(const vector<vector<int>> &f, const vector<int> &v) {
    vector<int> w;
    for (int y : v) {
        w.insert(w.end(), ALL(f[y]));
    }
    sort(ALL(w));
    w.erase(unique(ALL(w)), w.end());
    return w;
}

int64_t solve(int n, int k, const std::vector<int> &r, const vector<int> &c) {
    vector<vector<int>> f(n);
    REP (i, k) {
        f[r[i]].push_back(c[i]);
    }
    vector<int> v(n);
    iota(ALL(v), 0);

    int ans = 0;
    while (not v.empty()) {
        auto w = mult(f, v);
        ans += 1;
        if (v == w) return -1;
        v = w;
    }
    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;
}