#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
typedef long long ll;
using namespace std;
const int INF = 2e9;

int main() {
    cin.tie(0)->sync_with_stdio(0);

    int N, M;
    cin >> N >> M;
    vector D(N, vector<int>(M + 1, INF));
    rep(i, N) rep(j, M) cin >> D[i][j];
    rep(i, N) sort(D[i].begin(), D[i].end());

    int ans = INF;
    rep(i, M) {
        if (i >= M) return INF;
        int mi = D[0][i];

        int cur = mi;
        for (int j = 1; j < N; ++j) {
            cur = *lower_bound(D[j].begin(), D[j].end(), cur);
        }

        if (cur == INF) continue;
        ans = min(ans, cur - mi);
    }

    if (ans == INF) ans = -1;
    cout << ans << "\n";

    return 0;
}