/* -*- coding: utf-8 -*-
 *
 * 2463.cc:  No.2463 繧ｹ繝医Ξ繝ｼ繝医ヵ繝ｩ繝・す繝･ - yukicoder
 */

#include<cstdio>
#include<vector>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 500;
const int MAX_M = 500;
const int MAX_NM = MAX_N * MAX_M;
const int INF = 1 << 30;

/* typedef */

using vi = vector<int>;

/* global variables */

int rs[MAX_NM], ss[MAX_NM];
vi avs[MAX_M];
bool used[MAX_N + 1];

/* subroutines */

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  int nm = n * m;

  for (int i = 0; i < nm; i++) {
    scanf("%d%d", rs + i, ss + i), rs[i]--, ss[i]--;
    avs[ss[i]].push_back(rs[i]);
  }

  int minop = INF;
  for (int si = 0; si < m; si++) {
    fill(used, used + n + 1, false);
    int l = -1, r = -1, x = -1;
    for (auto ri: avs[si]) {
      used[ri] = true;
      int l0 = ri, r0 = ri;
      while (l0 > 0 && used[l0 - 1] && r0 - (l0 - 1) <= 4) l0--;
      while (r0 < n && used[r0 + 1] && (r0 + 1) - l0 <= 4) r0++;
      if (r0 - l0 == 4) { l = l0, r = r0, x = ri; break; }

      if (ri == 0) {
	used[n] = true;
	l0 = n, r0 = n;
	while (l0 > 0 && used[l0 - 1] && r0 - (l0 - 1) <= 4) l0--;
	if (r0 - l0 == 4) { l = l0, r = r0, x = n; break; }
      }
    }
    //printf(" si=%d, %d,%d,%d\n", si, l, r, x);

    int op = 0, c = 0;
    for (int i = 0; i < 5; i++)
      if (ss[i] == si &&
	  ((l <= rs[i] && rs[i] <= r) || (rs[i] == 0 && r == n))) c++;
    for (int i = 5; c < 5 && i < nm;) {
      op++;
      int d = 5 - c;
      while (d > 0) {
	if (ss[i] == si &&
	    ((l <= rs[i] && rs[i] <= r) || (rs[i] == 0 && r == n)))
	  c++;
	d--, i++;
      }
    }

    minop = min(minop, op);
  }

  printf("%d\n", minop);
  
  return 0;
}