#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

ll N, M;
vector<vector<ll>> D;

bool solve(ll c){
    ll n;
    vector<ll> q;
    for (int i=0; i<M; i++) q.push_back(D[0][i]);
    for (int i=1; i<N; i++){
        vector<ll> p;
        for (int j=0; j<M; j++){
            n = upper_bound(q.begin(), q.end(), D[i][j]) - lower_bound(q.begin(), q.end(), D[i][j]-c);
            if (n) p.push_back(D[i][j]);
        }
        swap(p, q);
    }
    return q.size() > 0;
}

int main(){

    cin >> N >> M;
    if (N == 1){
        cout << 0 << endl;
        return 0;
    }
    D.resize(N,  vector<ll>(M));
    for (int i=0; i<N; i++){
        for (int j=0; j<M; j++) cin >> D[i][j];
        sort(D[i].begin(), D[i].end());
    }

    ll l=-1, r=1e9, c;
    while(r-l>1){
        c = (l+r)/2;
        if (solve(c)) r=c;
        else l=c;
    }

    cout << (r == 1e9 ? -1 : r) << endl;

    return 0;
}