#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  VVI pos(m, VI(n));
  rep(i, n * m) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    pos[y][x] = i;
  }
  int ans = 1001001001;
  for(auto& p: pos) {
    p.emplace_back(p[0]);
    rep(i, n - 3) {
      static VI d;
      d.clear();
      rep(j, 5) d.emplace_back(p[i + j]);
      sort(all(d));
      int now = 0;
      int res = 0;
      rep(j, 5) {
        int x = d[j];
        if (now <= x) {
          int cnt = (x + 1 - now + 4 - j) / (5 - j);
          res += cnt;
          now += (5 - j) * cnt;
        }
      }
      chmin(ans, res);
    }
  }
  cout << ans - 1 << '\n';
}