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

#define INF 1000000005

int main() {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> d(n, vector<int>(m));
    vector<vector<int>> nxt(n, vector<int>(m, -1));
    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());
    }

    for (int i = 0; i < n - 1; i++) {
        int j = 0;
        for (int l = 0; l < m; l++) {
            while (j < m && d[i][j] <= d[i + 1][l]) {
                nxt[i][j] = l;
                j++;
            }
        }
    }

    int ans = INF;
    for (int i = 0; i < m; i++) {
        int now = INF;
        int k = i;
        for (int j = 0; j < n - 1; j++) {
            if (nxt[j][k] == -1) {
                now = INF;
                break;
            }
            now = min(now, d[j + 1][nxt[j][k]] - d[j][k]);
            k = nxt[j][k];
        }
        ans = min(ans, now);
    }

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