#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000000;
int main(){
  int N, M;
  cin >> N >> M;
  vector<vector<int>> D(N, vector<int>(M));
  for (int i = 0; i < N; i++){
    for (int j = 0; j < M; j++){
      cin >> D[i][j];
    }
  }
  vector<pair<int, int>> P(N * M);
  for (int i = 0; i < N; i++){
    for (int j = 0; j < M; j++){
      P[i * M + j] = make_pair(D[i][j], i);
    }
  }
  sort(P.begin(), P.end());
  int tv = INF, fv = -1;
  while (tv - fv > 1){
    int mid = (tv + fv) / 2;
    vector<bool> dp(N * M, false);
    vector<int> S(N, 0);
    int p = 0;
    for (int i = 0; i < N * M; i++){
      while (P[i].first - P[p].first > mid){
        if (dp[p]){
          S[P[p].second]--;
        }
        p++;
      }
      if (P[i].second == 0){
        dp[i] = true;
      } else {
        if (S[P[i].second - 1] > 0){
          dp[i] = true;
        }
      }
      if (dp[i]){
        S[P[i].second]++;
      }
    }
    bool ok = false;
    for (int i = 0; i < N * M; i++){
      if (P[i].second == N - 1 && dp[i]){
        ok = true;
      }
    }
    if (ok){
      tv = mid;
    } else {
      fv = mid;
    }
  }
  if (tv == INF){
    cout << -1 << endl;
  } else {
    cout << tv << endl;
  }
}