#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>

#define ll long long int
using namespace std;
int n, m, k;
const int inf = 10005;
pair<int, int> nul = {-1, -1};

void bfs(pair<int,int> x,  vector<pair<int,int>>& g, map<pair<int,int>,int>& path) {
    queue<pair<int,int>> q;
    map<pair<int, int>,int> used;
    for(int i = 0; i < n * m; i++) {
        used[g[i]];
        used[g[i]] = false;
    }
    map<pair<int,int>, pair<int,int>> parent;
    for(int i = 0; i < n * m; i++) {
        parent[g[i]];
        parent[g[i]] = nul;
    }

    q.push(x);
    used[x] = true;
    path[x] = 0;
    while(!q.empty()) {
        pair<int,int> v = q.front(); q.pop();
        int dx[4] = {-1, 0, 1, 0};
        int dy[4] = {0, 1, 0, -1};
        for(int i = 0; i < 4; i++) {
            pair<int,int> s = {v.first + dx[i], v.second + dy[i]};
            if(path.find(s) != path.end()) {
                if(!used[s] && path[s] > path[v] + 1) {
                    path[s] = path[v] + 1;
                    q.push(s);
                    used[s] = true;
                    parent[s] = v;
                }
            }
        }
    }
}

int main() {
    cin >> n >> m >> k;
    vector<pair<int, int>> init;
    for (int i = 0; i < k; i++) {
        int x, y;
        cin >> x >> y;
        init.push_back({x, y});
    }

    vector<pair<int, int>> g;
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            g.push_back({i, j});
        }
    }

    map<pair<int,int>,int> path;
    for(int i = 0; i < n * m; i++) {
        path[g[i]];
        path[g[i]] = inf;
    }

    for(int i = 0; i < k; i++) {
        bfs(init[i], g, path);
    }

    int ans = 0;
    for(int i = 0; i < n * m; i++) {
        if(path[g[i]] > ans) ans = path[g[i]];
    }

    cout << ans;
    return 0;
}