#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    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];
        }
        sort(D[i].begin(), D[i].end());
    }
    constexpr int INF = 1<<30;
    vector<vector<int>> dp(N, vector<int>(M, INF));
    for( int i = 0; i < N-1; i++ ) {
        for( int j = 0; j < M; j++ ) {
            for( int k = 0; k < M; k++ ) {
                if( D[i+1][k] < D[i][j] || ( i > 0 && dp[i][j] == INF ) ) continue;
                dp[i+1][k] = min({dp[i+1][k], dp[i][j], abs(D[i+1][k]-D[i][j])});
            }
        }
    }
    int me = *min_element(dp[N-1].begin(), dp[N-1].end());
    cout << ( me == INF ? -1 : me ) << endl;
}