#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_set>
using namespace std;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
#endif
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i, n) _rep2(i, 0, n)
#define _rep2(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using i64 = long long;
template <class T, class U>
bool chmin(T& a, const U& b) {
    return (b < a) ? (a = b, true) : false;
}
template <class T, class U>
bool chmax(T& a, const U& b) {
    return (b > a) ? (a = b, true) : false;
}
inline void YesNo(bool f, string yes = "Yes", string no = "No") {
    std::cout << (f ? yes : no) << "\n";
}

template <typename T>
istream& operator>>(istream& i, vector<T>& v) {
    rep(j, v.size()) i >> v[j];
    return i;
}
template <typename T>
string join(vector<T>& v) {
    stringstream s;
    rep(i, v.size()) s << ' ' << v[i];
    return s.str().substr(1);
}
template <typename T>
ostream& operator<<(ostream& o, vector<T>& v) {
    if (v.size()) o << join(v);
    return o;
}
template <typename T>
string join(vector<vector<T>>& vv) {
    string s = "\n";
    rep(i, vv.size()) s += join(vv[i]) + "\n";
    return s;
}
template <typename T>
ostream& operator<<(ostream& o, vector<vector<T>>& vv) {
    if (vv.size()) o << join(vv);
    return o;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    if (n == 1) {
        cout << 0 << endl;
        return 0;
    }
    vector d(n, vector<i64>(m));
    rep(i, n) cin >> d[i];
    rep(i, n) sort(all(d[i]));
    vector dp(n + 1, vector<i64>(m, 1e18));
    rep(i, m) dp[1][i] = 0;
    rep(i, 1, n) {
        rep(j, m) {
            int l = upper_bound(all(d[i - 1]), d[i][j]) - d[i - 1].begin();
            if (l == 0) continue;
            l--;
            for (int k = l; k >= 0; k--) {
                if (dp[i + 1][j] <= d[i][j] - d[i - 1][k]) break;
                if (dp[i + 1][j] <= dp[i][k]) continue;
                chmin(dp[i + 1][j], max(d[i][j] - d[i - 1][k], dp[i][k]));
            }
        }
    }
    auto ans = *min_element(all(dp[n]));
    cout << (ans == 1e18? -1: ans) << endl;
}