#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef int64_t lint;
#define rep(i, n) for(int i=0; i<n; i++)
#define repx(i, l, n) for(int i=l; i<n; i++)
#define all(v) v.begin(), v.end()
#define show(x) cout << #x << ": " << x << endl;
#define list(x) cout << #x << ": " << x << "  ";
#define pb push_back
using vi = vector<lint>;
using vvi = vector<vector<lint>>;
template<class T> inline void vin(vector<T>& v) { rep(i, v.size()) cin >> v.at(i); }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<class T> inline void drop(T x) { cout << x << endl; exit(0); }
template<class T> void vout(vector<T> v) { rep(i, v.size()) { cout << v.at(i) << ' '; } cout << endl; }
constexpr lint LINF = LLONG_MAX/2;

int main() {
	lint N, M;
	cin >> N >> M;
	vvi v(N, vi(M));
	rep(i, N) vin(v[i]);
	rep(i, N) sort(all(v[i]));

	lint a=LINF, b=0, c=0, x, y, z;
	vvi dp(N, vi(M, LINF));
	rep(i, M) dp[0][i] = 0;
	rep(i, N-1) {
		rep(j, M) {
			rep(k, M) {
				if (v[i+1][j]-v[i][k] < 0) break;
				chmin(dp[i+1][j], max(dp[i][k], v[i+1][j]-v[i][k]));
				if (dp[i+1][j] == 0) break;
			}
		}
	}
	rep(i, M) chmin(a, dp[N-1][i]);
	if (a == LINF) std::cout << -1 << '\n';
	else std::cout << a << '\n';
}