#include 
#include 
#include 
using namespace std;

int main()
{
    int L, W;
    cin >> L >> W;
    
    int treba;
    cin >> treba;
    
    
    int n;
    cin >> n;
    
    vector> v(n);
    
    vector> visited(L, vector(W, false));
    for(int i = 0; i < n; i++) {
        cin >> v[i].first >> v[i].second;
        v[i].first--;
        v[i].second--;
        
        visited[v[i].first][v[i].second] = true;
    }
    
    queue>> q;
    queue q_mesec;
    
    treba -= n;
    
    q.push(v);
    q_mesec.push(0);
    
    int di[] = {-1, 1, 0, 0};
    int dj[] = {0, 0, -1, 1};
    while(!q.empty()) {
        vector> c = q.front();
        q.pop();
        int mesec = q_mesec.front();
        q_mesec.pop();
        
        if(treba <= 0) {
            cout << mesec << endl;
            break;
        }
        
        vector> t;
        for(int i = 0; i < c.size(); i++) {
            for(int j = 0; j < 4; j++) {
                int ti = c[i].first + di[j];
                int tj = c[i].second + dj[j];
                
                if(ti >= 0 and ti < L and tj >= 0 and tj < W and !visited[ti][tj]) {
                    visited[ti][tj] = true;
                    treba--;
                    t.push_back(make_pair(ti, tj));
                }
            }
        }
        q.push(t);
        q_mesec.push(mesec + 1);
    }
    
    

    
    return 0;

}