#pragma GCC target("avx2")
#pragma GCC optimize("unroll-loops")

#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstring>
#include <deque>
#include <forward_list>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define ALL(x) (x).begin(), (x).end()
#define PC(x) __builtin_popcount(x)
#define PCL(x) __builtin_popcountll(x)

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
template <class T>
bool chmax(T& a, const T& b)
{
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <class T>
bool chmin(T& a, const T& b)
{
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}
const double PI = 3.14159265358979323846;
const double PI2 = PI * 2.0;
const double EPS = 1E-09;
const ll MOD = 1E+09 + 7; // =998244353;
const ll INFL = 1E18;
const int INFI = 1E09;
const int MAX_N = 1500;
struct edge {
    int to, cost, id;
};

ll di[4] = { 0, -1, 0, 1 }, dj[4] = { 1, 0, -1, 0 };

int N, M;

ll D[MAX_N + 1][MAX_N + 1];

bool check(ll x)
{
    vector<bool> dp(M + 2, false);
    vector<int> s(M + 2, 0);
    for (int j = 1; j <= M; ++j) {
        s[j] = j;
    }
    for (int i = 2; i <= N; ++i) {
        for (int j = 1; j <= M; ++j) {
            int left = lower_bound(D[i - 1] + 1, D[i - 1] + M + 1, D[i][j] - x) - D[i - 1];
            int right = (int)(upper_bound(D[i - 1] + 1, D[i - 1] + M + 1, D[i][j]) - D[i - 1]);
            int cnt = s[right - 1] - s[left - 1];
            if (cnt > 0) {
                dp[j] = true;
            } else {
                dp[j] = false;
            }
        }
        for (int j = 1; j <= M; ++j) {
            s[j] = (dp[j] ? 1 : 0);
        }
        for (int j = 1; j <= M; ++j) {
            s[j] += s[j - 1];
        }
    }
    for (int j = 1; j <= M; ++j) {
        if (dp[j])
            return true;
    }
    return false;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> N >> M;

    for (int i = 1; i <= N; ++i) {
        for (int j = 1; j <= M; ++j) {
            cin >> D[i][j];
        }
        sort(D[i], D[i] + M + 1);
    }

    if (N == 1) {
        cout << 0 << "\n";
        return 0;
    }

    ll low = -1, high = INFI;
    while (high - low > 1) {
        ll mid = (low + high) / 2;
        if (check(mid)) {
            high = mid;
        } else {
            low = mid;
        }
    }

    // for (int i = 0; i < N; i++) {
    //     for (int j = 0; j < N; j++) {
    //         cout << "i = " << i << ", j = " << j << ", dp = " << dp[i][j] << "\n";
    //     }
    // }

    cout << (high == INFI ? -1 : high) << "\n";

    return 0;
}