#include <stdio.h>

int main () {
  int n = 0;
  int m = 0;
  int ni = 0;
  int mi = 0;
  
  int res = 0;
  
  int ans = 0;
  int pos[500][500] = {};
  int tmp[5] = {};
  
  res = scanf("%d", &n);
  res = scanf("%d", &m);
  for (int i = 0; i < n*m; i++) {
    res = scanf("%d", &ni);
    res = scanf("%d", &mi);
    pos[mi-1][ni-1] = i;
  }
  
  ans = n*m;
  for (int i = 0; i < m; i++) {
    for (int j = 0; j < n-3; j++) {
      int tmpcnt = 0;
      int tmppos = 4;
      for (int k = 0; k < 5; k++) {
        tmp[k] = pos[i][(j+k)%n];
      }
      for (int k = 4; k > 0; k--) {
        for (int l = 0; l < k; l++) {
          if (tmp[l] > tmp[l+1]) {
            int s = tmp[l];
            tmp[l] = tmp[l+1];
            tmp[l+1] = s;
          }
        }
      }
      tmpcnt = tmp[0]/5;
      tmppos = (1+tmp[0]/5)*5-1;
      if (tmppos < tmp[1]) {
        tmpcnt += 1+(tmp[1]-tmppos-1)/4;
        tmppos += (1+(tmp[1]-tmppos-1)/4)*4;
      }
      if (tmppos < tmp[2]) {
        tmpcnt += 1+(tmp[2]-tmppos-1)/3;
        tmppos += (1+(tmp[2]-tmppos-1)/3)*3;
      }
      if (tmppos < tmp[3]) {
        tmpcnt += 1+(tmp[3]-tmppos-1)/2;
        tmppos += (1+(tmp[3]-tmppos-1)/2)*2;
      }
      if (tmppos < tmp[4]) {
        tmpcnt += tmp[4]-tmppos;
        tmppos = tmp[4];
      }
      if (tmpcnt < ans) {
        ans = tmpcnt;
      }
    }
  }
  
  printf("%d\n", ans);
  return 0;
}