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

#define INF 1000000001

int main() {
    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());
    }

    int ng = -1, ok = INF;

    while (ok - ng > 1) {
        int key = (ok + ng) / 2;
        vector<bool> dp(m, true);

        for (int i = 0; i < n - 1; i++) {
            vector<bool> ndp(m, false);
            int l = 0;
            for (int j = 0; j < m; j++) {
                if (!dp[j]) continue;
                while (l < m && d[i + 1][l] < d[i][j]) {
                    l++;
                }
                while (l < m && d[i][j] <= d[i + 1][l] && d[i + 1][l] <= d[i][j] + key) {
                    ndp[l] = true;
                    l++;
                }
            }
            dp.swap(ndp);
        }

        bool flag = false;

        for (int i = 0; i < m; i++) {
            if (dp[i]) {
                flag = true;
                break;
            }
        }

        if (flag) ok = key;
        else ng = key;
    }

    cout << (ok == INF ? -1 : ok) << endl;
}