#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <tuple>
#include <cstdint>
#include <cstdio>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <deque>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <cctype>
#include <climits>
#include <functional>
#include <cassert>
#include <numeric>
#define rep(i, n) for(int i = 0; i < (n); i++)
#define per(i, n) for(int i = (n) - 1; i >= 0; i--)
using ll = long long;
#define vi vector<int>
#define vvi vector<vi>
#define vl vector<ll>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
constexpr int mod = 1000000007;
using namespace std;
template<class T, class U>
bool chmax(T &a, const U &b){ return a < b ? (a = b, 1) : 0; }
template<class T, class U>
bool chmin(T &a, const U &b){ return a > b ? (a = b, 1) : 0; }
int main(){
  int n,m;
  cin >> n >> m;
  vvi d(n, vi(m));
  rep(i, n){
    rep(j, m) cin >> d[i][j];
    sort(all(d[i]));
  }
  int left = -1, right = mod;
  while(left + 1 < right){
    const int mid = (left + right) / 2;
    vi ok(m, 1);
    for(int i = 1; i < n; i++){
      vi nxt(m + 1);
      rep(j, m) if(ok[j]){
        const int l = lower_bound(all(d[i]), d[i-1][j]) - d[i].begin();
        const int r = upper_bound(all(d[i]), d[i-1][j] + mid) - d[i].begin();
        nxt[l]++;
        nxt[r]--;
      }
      rep(j, m) nxt[j+1] += nxt[j];
      swap(ok, nxt);
    }
    bool isok = false;
    rep(i, m) if(ok[i]){
      isok = true; break;
    }
    if(isok) right = mid;
    else left = mid;
  }
  if(right == mod) cout << "-1\n";
  else cout << right << "\n";
}