#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());

    auto f = [&](int i) -> int {
        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) return INF;
        return cur - mi;
    };

    ll l = 0, r = M;
    while (r - l > 2) {
        ll c1 = (l * 2 + r) / 3;
        ll c2 = (l + r * 2) / 3;
        if (f(c1) > f(c2))
            l = c1;
        else
            r = c2;
    }

    int ans = min({f(l), f(l + 1), f(l + 2)});
    if (ans == INF) ans = -1;
    cout << ans << "\n";

    return 0;
}