#include <bits/stdc++.h>
using namespace std;

int main() {
	int n, m;
	cin >> n >> m;
	vector<pair<int, int>> D(n * m);
	vector<vector<int>> nm(m, vector<int>(n));
	for (int i = 0; i < n * m; i++) {
		int u, v;
		cin >> u >> v;
		u--;
		v--;
		nm[v][u] = max(i - 4, 0);
		D[i] = make_pair(v, u);
	}
	int ans = INT_MAX;
	for (int i = 0; i < m; i++) {
		for (int j = 0; j < n - 3; j++) {
			set<pair<int, int>> mem;
			int k = j;
			do {
				mem.insert(make_pair(i, k));
				(++k) %= n;
			} while (k != (j + 5) % n);
			set<pair<int, int>> cd;
			for (pair<int, int> x : mem) {
				if (nm[x.first][x.second] == 0) {
					cd.insert(x);
				}
			}
			int time = 5;
			int res = 0;
			while (mem != cd) {
				int add = min(n * m - time, 5 - (int)cd.size());
				for (int k = 0; k < add; k++) {
					if (mem.count(D[time + k])) {
						cd.insert(D[time + k]);
					}
				}
				time += add;
				res++;
			}
			ans = min(ans, res);
		}
	}
	cout << ans << endl;
}