#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
struct elem
{
    int i;
    int j;
};

int isinfected(int **arr, int N, int M)
{
    int k = 1, i = 0, j = 0;
    for (i = 1; i < N - 1; i++)
        for(j = 1; j < M - 1 ; j++)
            k = k * arr[i][j];
    return k;
}
int main (void)
{
    int N, M, k, i, x, j = 0, time = 1;
    scanf("%d%d", &N, &M);
    int **colony = malloc(sizeof(int *) * (N + 2));
    for (i = 0; i < N + 2; i++)
    {
        colony[i] = malloc(sizeof(int) * (M + 2));
    }
    for (i = 1; i < N + 1; i++)
        for(j = 1; j < M + 1; j++)
            colony[i][j] = 0;
    scanf("%d", &k);
    struct elem *viruses = malloc(sizeof(struct elem) * k);
    for( x = 0; x < k; x++)
    {
        scanf("%d%d", &(viruses[x].j), &(viruses[x].i));
    }
    if (N * M == k)
        printf("%d", 0);
    else{
    for (i = 1; i < N + 1; i++)
        for(j = 1; j < M + 1; j++)
            for( x = 0; x < k; x++)
                if (i == viruses[x].i  && j == viruses[x].j)
                    colony[i][j] = 1;
    while (isinfected(colony, N + 2, M + 2) != 1)
    {
        for(i = 1; i < N + 1; i++)
        {
            if (isinfected(colony, N + 2, M + 2))
            {
                printf("%d", time);
                return 0;
            }
            for(j = 1; j < M + 1; j++)
            {
                if ( colony[i][j] == time)
                {
                    if (colony [i][j] != colony[i - 1][j] )
                         colony[i - 1][j] = 1 + colony[i][j];
                    if (colony [i][j] != colony[i][j - 1] )
                         colony[i][j - 1] = 1 + colony[i][j];
                    if (colony [i][j] != colony[i][j + 1] )
                         colony[i][j + 1] = 1 + colony[i][j];
                    if (colony [i][j] != colony[i + 1][j] )
                        colony[i + 1][j] = 1 + colony[i][j];


                }

            }



        }
        time++;
    }
    }

    return 0;
}