import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 1001001001;

void main() {
  try {
    for (; ; ) {
      const N = readInt;
      const M = readInt;
      auto D = new int[][](N, M);
      foreach (i; 0 .. N) {
        foreach (j; 0 .. M) {
          D[i][j] = readInt;
        }
        D[i].sort;
      }
      
      auto dp = new bool[][](N, M);
      bool check(int thr) {
        foreach (i; 0 .. N) {
          dp[i][] = false;
        }
        dp[0][] = true;
        foreach (i; 0 .. N - 1) {
          int j0;
          int mx = -INF;
          foreach (j1; 0 .. M) {
            for (; j0 < M && D[i][j0] <= D[i + 1][j1]; ++j0) {
              if (dp[i][j0]) {
                chmax(mx, D[i][j0]);
              }
            }
            if (mx >= 0 && D[i + 1][j1] - mx <= thr) {
              dp[i + 1][j1] = true;
            }
          }
        }
        return dp[N - 1].any;
      }
      
      int lo = -1, hi = 10^^9 + 1;
      if (check(hi)) {
        for (; lo + 1 < hi; ) {
          const mid = (lo + hi) / 2;
          (check(mid) ? hi : lo) = mid;
        }
        writeln(hi);
      } else {
        writeln(-1);
      }
    }
  } catch (EOFException e) {
  }
}