#include <bits/stdc++.h>

using namespace std;
 int L , W;

bool valid(int a, int b)
{
    if(a>=0 && b>=0 && a < L && b < W )
        return 1;
    return 0;
}

int main()
{

    cin >> L >> W;
    int N;
    cin >> N;
    int B;
    cin >> B;
    queue <pair <int,int> >verts;
    int graph[L+1][W+1];
    graph[L][W]=999999;
    for(int i=0;i<B;i++)
    {
        int x,y;
        cin >> x >> y;
        verts.push(make_pair(x-1,y-1));
        graph[x-1][y-1]=0;
    } int newB=B;

    while(!verts.empty())
    {

        pair <int,int> vert=verts.front();
        verts.pop();
        if(valid(vert.first+1,vert.second)&&graph[vert.first+1][vert.second]>graph[vert.first][vert.second]+1)
        {
            graph[vert.first+1][vert.second]=graph[vert.first][vert.second]+1;

            verts.push(make_pair(vert.first+1,vert.second));
            newB++;


        }
        if(valid(vert.first-1,vert.second)&&graph[vert.first-1][vert.second]>graph[vert.first][vert.second]+1)
        {
            graph[vert.first-1][vert.second]=graph[vert.first][vert.second]+1;

            verts.push(make_pair(vert.first-1,vert.second));
            newB++;

        }
        if(valid(vert.first,vert.second+1)&&graph[vert.first][vert.second+1]>graph[vert.first][vert.second]+1)
        {
            graph[vert.first][vert.second+1]=graph[vert.first][vert.second]+1;

            verts.push(make_pair(vert.first,vert.second+1));
            newB++;

        }
        if(valid(vert.first,vert.second-1)&&graph[vert.first][vert.second-1]>graph[vert.first][vert.second]+1)
        {
            graph[vert.first][vert.second-1]=graph[vert.first][vert.second]+1;
            verts.push(make_pair(vert.first,vert.second-1));
            newB++;

        }
        if(newB>=N)
        {
            cout << graph[vert.first][vert.second]+1;
            return 0;
        }


    }

//for(int i=0;i<L;i++)
//{
//    for(int j=0;j<W;j++)
//    {cout << graph[i][j] << " ";
//    }
//    cout << endl;
//}
}