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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, m;
    cin >> n >> m;
    vector<vector<int>> A(m, vector<int>(n));
    vector<pair<int,int>> a(n * m);
    for(int i = 0; i < n * m; i++){
        int u, v;
        cin >> u >> v;
        u--, v--;
        a[i] = make_pair(u, v);
        A[v][u] = i + 1;
    }
    int ans = 1 << 30;
    auto f = [&](vector<int> b){
        sort(b.begin(), b.end());
        int cur = 5, s = 0;
        for(int i = 0; i < 5; i++){
            if(cur >= b[i]) continue;
            int d = (b[i] - cur + 4 - i) / (5 - i);
            cur += d * (5 - i);
            s += d; 
        }
        return s;
    };
    vector<int> c(5);
    for(int i = 0; i < m; i++){
        for(int j = 0; j + 5 <= n; j++){
            for(int k = 0; k < 5; k++){
                c[k] = A[i][j + k];
            }
            ans = min(ans, f(c));
        }
        c[0] = A[i][0];
        for(int j = 0; j < 4; j++){
            c[j + 1] = A[i][n - 1 - j];
        }
        ans = min(ans, f(c));
    }
    cout << ans << '\n';
}