#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll,ll>;
#define fix(x) fixed << setprecision(x)
#define asc(x) x, vector<x>, greater<x>
#define rep(i, n) for(ll i = 0; i < n; i++)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL <<62);
constexpr int INF = (1<<30);
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n,m;
    cin >> n >> m;
    vector<vector<int>> a(n,vector<int>(m));
    rep(i,n){
        rep(j,m) cin >> a[i][j];
        sort(all(a[i]));
        a[i].push_back(INF);
    }
    int ans = INF;
    rep(i,m){
        int upd = 0, now = a[0][i];
        for(int j=1;j<n;j++){
            int x = *lower_bound(all(a[j]), now);
            chmax(upd, x-now);
            now = x;
        }
        if(now!=INF) chmin(ans, upd);
    }
    if(ans==INF) ans = -1;
    cout << ans << '\n';
    return 0;
}