#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  VVI d(n, VI(m));
  rep(i, n) rep(j, m) cin >> d[i][j];
  rep(i, n) sort(all(d[i]));
  constexpr int INF = 1001001001;
  int l = -1, r = INF;
  while(r - l > 1) {
    int c = (l + r) / 2;
    vector<char> dp(m, true), ndp(m);
    for(int i = 1; i < n; i++) {
      int j_from = 0;
      int mx = -INF;
      rep(j, m) {
        while(j_from < m && d[i-1][j_from] <= d[i][j]) {
          if (dp[j_from]) mx = d[i-1][j_from];
          j_from++;
        }
        ndp[j] = d[i][j] - mx <= c;
      }
      swap(dp, ndp);
    }
    (accumulate(all(dp), 0) ? r : l) = c;
  }
  if (r == INF) r = -1;
  cout << r << '\n';
}