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


int main() {
  int n, m; cin >> n >> m;

  vector d(n, vector(m, 0));

  for(auto&&v : d){
    for(auto&&e : v) cin >> e;
    sort(begin(v), end(v));
  }

  auto isok = [&](int x) -> bool {
    vector<int> ok(m + 1); iota(begin(ok), end(ok), 0);

    for(int i = 0; i < n - 1; ++i){
      vector<int> nx(m + 1);

      for(int j = 0; j < m; ++j){
        int l = lower_bound(begin(d[i]), end(d[i]), d[i+1][j] - x) - begin(d[i]);
        int r = upper_bound(begin(d[i]), end(d[i]), d[i+1][j]) - begin(d[i]);
        nx[j + 1] = nx[j] + ((ok[r] - ok[l]) > 0);
      }

      ok = nx;
    }
    return ok[m] > 0;
  };

  int inf = 1e9 + 1;
  int ok = inf, ng = -1;
  while(abs(ok-ng) > 1){
    int wj = (ok + ng)/2;
    if(isok(wj)) ok = wj;
    else ng = wj;
  }

  if(ok == inf) ok = -1;
  cout << ok << "\n";

}