#include<bits/stdc++.h>
using namespace std;
//#pragma GCC optimize("Ofast")

#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}


int main(){
    ll n,m;cin >> n >> m;
    vvl mat(n,vl(m));
    rep(_,n*m){
        ll a,b;cin >> a >> b;a--;b--;
        mat[a][b]=_;
    }
    ll ans=INF;
    vl use(n*m);
    auto solve=[&](vl v)->ll {
        rep(i,5)use[v[i]]++;
        ll top=0;
        ll cnt=0;
        ll hand=5;
        while(hand){
            ll f=(v[5-hand]-top)/hand;
            cnt+=f;top+=f*hand;
            //
            cnt++;
            ll cur=0;
            for(ll j=top;j<top+hand;j++){
                if(use[j])cur++;
            }
            top+=hand;
            hand-=cur;
        }
        rep(i,5)use[v[i]]--;
        return cnt;
    };
    rep(j,m){
        for(ll i=0;i<n-4;i++){
            vl cand;
            for(ll k=0;k<5;k++)cand.emplace_back(mat[i+k][j]);
            sort(all(cand));
            chmin(ans,solve(cand));
        }
        vl cand={mat[0][j],mat[n-1][j],mat[n-2][j],mat[n-3][j],mat[n-4][j]};
        sort(all(cand));
        chmin(ans,solve(cand));
    }
    cout << ans-1 << endl;
}