#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 998244353;
// constexpr int MOD = 1000000007;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  constexpr int D = 1000000000;
  int n, m; cin >> n >> m;
  vector d(n, vector(m, 0));
  REP(i, n) {
    REP(j, m) cin >> d[i][j];
    sort(ALL(d[i]));
  }
  int lb = -1, ub = D;
  while (ub - lb > 1) {
    const int f = (lb + ub) / 2;
    vector<int> dp(m, true), nxt(m);
    FOR(i, 1, n) {
      fill(ALL(nxt), false);
      int l = 0, r = 0, num = 0;
      REP(j, m) {
        for (; r < m && d[i - 1][r] <= d[i][j]; ++r) num += dp[r];
        for (; l < r && d[i - 1][l] + f < d[i][j]; ++l) num -= dp[l];
        if (num > 0) nxt[j] = true;
      }
      dp.swap(nxt);
    }
    (count(ALL(dp), true) > 0 ? ub : lb) = f;
  }
  cout << (ub == D ? -1 : ub) << '\n';
  return 0;
}