#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i=0; i<n; i++)
#define pb push_back
typedef long long ll;

int main() {
    cin.tie(0); ios::sync_with_stdio(false);
    
    int N, M; cin >> N >> M;
    vector<int> D[N];
    rep(i, N) rep(j, M) {
        int d; cin >> d;
        D[i].pb(d);
    }
    rep(i, N) sort(D[i].begin(), D[i].end());
    
    int l = 0;
    int r = 1000000010;
    
    while (l<=r) {
        int m = (l+r)/2;
        
        bool dp[M];
        rep(i, M) dp[i] = true;
        
        for (int i=1; i<N; i++) {
            vector<int> acc = {0};
            rep(j, M) acc.pb(acc.back()+(dp[j] ? 1 : 0));
            
            bool ndp[M];
            rep(j, M) ndp[j] = false;
            
            rep(j, M) {
                int le = lower_bound(D[i-1].begin(), D[i-1].end(), D[i][j]-m)-D[i-1].begin();
                int ri = upper_bound(D[i-1].begin(), D[i-1].end(), D[i][j])-D[i-1].begin();
                if (acc[ri]-acc[le]>0) ndp[j] = true;
            }
            
            rep(j, M) dp[j] = ndp[j];
        }
        
        bool ok = false;
        rep(i, M) if (dp[i]) ok = true;
        if (ok) r = m-1;
        else l = m+1;
    }
    
    int ans = l;
    if (ans>1000000010) cout << -1 << endl;
    else cout << ans << endl;
}