#include <stdio.h>
#include <stdlib.h>

#define INF 1000000000000000LL

int cmp_ll (const void *ap, const void *bp) {
  long long a = *(long long *)ap;
  long long b = *(long long *)bp;
  
  if (a < b) {
    return -1;
  }
  
  if (a > b) {
    return 1;
  }
  
  return 0;
}

int main () {
  int n = 0;
  int m = 0;
  long long d[1500][1500] = {};
  
  int res = 0;
  
  long long dp[1500][1501] = {};
  long long ans[2] = { -1LL, INF };
  
  res = scanf("%d", &n);
  res = scanf("%d", &m);
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      res = scanf("%lld", d[i]+j);
    }
    qsort(d[i], m, sizeof(long long), cmp_ll);
  }
  
  while (ans[1]-ans[0] > 1LL) {
    long long nxt = (ans[0]+ans[1])/2LL;
    for (int j = 0; j <= m; j++) {
      dp[0][j] = j;
    }
    for (int i = 1; i < n; i++) {
      int idx1 = 0;
      int idx2 = 0;
      for (int j = 0; j < m; j++) {
        while (idx1 < m && d[i][j]-nxt > d[i-1][idx1]) {
          idx1++;
        }
        while (idx2 < m && d[i][j] >= d[i-1][idx2]) {
          idx2++;
        }
        if (dp[i-1][idx2]-dp[i-1][idx1] > 0) {
          dp[i][j+1] = dp[i][j]+1;
        } else {
          dp[i][j+1] = dp[i][j];
        }
      }
    }
    if (dp[n-1][m] > 0) {
      ans[1] = nxt;
    } else {
      ans[0] = nxt;
    }
  }
  
  if (ans[1] >= INF) {
    printf("-1\n");
  } else {
    printf("%lld\n", ans[1]);
  }
  
  return 0;
}