#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll n,m;
    cin>>n>>m;
    vector<vector<ll>> d(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 ans=1e18;
    for(int i=0;i<m;i++){
        ll v=d[0][i];
        ll local=-1;
        bool flg=true;
        for(int j=1;j<n;j++){
            auto lb=lower_bound(d[j].begin(),d[j].end(),v);
            if(lb==d[j].end()){
                flg=false;
                break;
            }
            local=max(local,(*lb-v));
            v=(*lb);
        }
        if(local<0)continue;
        if(flg)ans=min(ans,local);
    }
    if(ans==1e18){
        cout<<-1<<endl;
    }else{
        cout<<ans<<endl;
    }
    return 0;
}