#include <bits/stdc++.h>
#include <atcoder/all>

using namespace std;
using namespace atcoder;

struct Fast {
  Fast() {
    std::cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << setprecision(10);
  }
} fast;

#define all(a) (a).begin(), (a).end()
#define contains(a, x) ((a).find(x) != (a).end())
#define rep(i, a, b) for (int i = (a); i < (int)(b); i++)
#define rrep(i, a, b) for (int i = (int)(b)-1; i >= (a); i--)
#define YN(b) cout << ((b) ? "YES" : "NO") << "\n";
#define Yn(b) cout << ((b) ? "Yes" : "No") << "\n";
#define yn(b) cout << ((b) ? "yes" : "no") << "\n";

template <typename T>
ostream& operator<<(ostream& os, vector<T>& vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  }
  return os;
}

using ll = long long;
using vb = vector<bool>;
using vvb = vector<vb>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;

using mint = modint998244353;
using vm = vector<mint>;
using vvm = vector<vm>;
inline ostream& operator<<(ostream& os, const mint P) { return os << P.val(); };

void solve() {
  int h, w;
  cin >> h >> w;
  vi x(w), y(w);
  rep(i, 0, w) cin >> x[i] >> y[i];

  int s = w * 2;
  mcf_graph<int, int> g(s + 2);
  rep(i, 0, w) g.add_edge(s, i, 1, 0);
  rep(i, 0, w) g.add_edge(w + i, s + 1, 1, 0);

  rep(i, 0, w) rep(j, 0, w) {
    int dx = x[i] - 1, dy = y[i] - (j + 1);
    if (dx < 0) dx = -dx, dy = -dy;
    int c = 0;
    while (dx > 0 && dy > 0) c++, dx--, dy--;
    dy = abs(dy);
    while (dx) c++, dx--;
    while (dy) c++, dy--;
    g.add_edge(i, j + w, 1, c);
  }

  cout << g.flow(s, s + 1).second << "\n";
}

int main() {
  int t = 1;
  // cin >> t;
  while (t--) solve();
}