#include <bits/stdc++.h>
using namespace std;

int main() {
    int64_t N, M;
    cin >> N >> M;

    vector<vector<int64_t>> D(N, vector<int64_t>(M));
    for (int64_t i = 0; i < N; i++) {
        for (int64_t j = 0; j < M; j++) {
            cin >> D[i][j];
        }
        sort(D[i].begin(), D[i].end());
    }

    // -1であるかどうか判定
    int64_t pre_value = 0;
    for (int64_t i = 0; i < N; i++) {
        auto itr = lower_bound(D[i].begin(), D[i].end(), pre_value);
        if (itr == D[i].end()) {
            cout << -1 << endl;
            return 0;
        }
        pre_value = *itr;
    }

    // 二分探索
    constexpr int64_t kInf = LLONG_MAX / 2;
    int64_t ok = kInf / 2, ng = -1;
    while (ok - ng != 1) {
        const int64_t mid = (ok + ng) / 2;
        set<int64_t> st;
        for (int64_t j = 0; j < M; j++) {
            st.insert(D[0][j]);
        }

        for (int64_t i = 1; i < N; i++) {
            set<int64_t> new_st;
            for (int64_t j = 0; j < M; j++) {
                auto itr = st.lower_bound(D[i][j] - mid);
                if (itr != st.end() && *itr <= D[i][j]) {
                    new_st.insert(*itr);
                }
            }
            st = new_st;
        }

        (st.empty() ? ng = mid : ok = mid);
    }

    cout << ok << endl;
}