#include <bits/stdc++.h>

using namespace std;

void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}

int main() {
    fast_io();
    int n, m;
    cin >> n >> m;
    vector<vector<int>> d(n, vector<int>(m));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> d[i][j];
        }
        sort(d[i].begin(), d[i].end());
    }
    {
        vector<int> dp(n);
        dp[0] = 0;
        for (int i = 1; i < n; i++) {
            dp[i] = lower_bound(d[i].begin(), d[i].end(), d[i - 1][dp[i - 1]]) -
                    d[i].begin();
            if (dp[i] == m) {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    int ng = -1, ok = 1e9;
    while (ok - ng > 1) {
        int mid = (ok + ng) / 2;
        vector<vector<bool>> dp(n, vector<bool>(m));
        vector<vector<int>> cum(n, vector<int>(m + 1));
        for (int i = 0; i < m; i++) {
            dp[0][i] = true;
            cum[0][i + 1] = cum[0][i] + d[0][i];
        }
        for (int i = 1; i < n; i++) {
            for (int j = 0; j < m; j++) {
                int lb = lower_bound(d[i - 1].begin(), d[i - 1].end(),
                                     d[i][j] - mid) -
                         d[i - 1].begin();
                int ub =
                    lower_bound(d[i - 1].begin(), d[i - 1].end(), d[i][j] + 1) -
                    d[i - 1].begin();
                if (cum[i - 1][ub] - cum[i - 1][lb] > 0) {
                    dp[i][j] = true;
                }
            }
            for (int j = 0; j < m; j++) {
                cum[i][j + 1] = cum[i][j] + dp[i][j];
            }
        }
        bool is_ok = false;
        for (int i = 0; i < m; i++) {
            if (dp[n - 1][i]) {
                is_ok = true;
                break;
            }
        }
        if (is_ok) {
            ok = mid;
        } else {
            ng = mid;
        }
    }
    cout << ok << endl;
}