#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <queue>
#include <ext/pb_ds/hash_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
typedef long long ll;

namespace pbds{
	using namespace __gnu_pbds;

	template <class T, class cmp = std::less<T>>
	using set = tree<T, null_type, cmp, rb_tree_tag, tree_order_statistics_node_update>;

	template <class key, class value, class cmp = std::less<key>>
	using map = tree<key, value, cmp, rb_tree_tag, tree_order_statistics_node_update>;
}

int n, m, k;
const int inf = 10005;

template <class VVP, class QP>
void bfs(int n, int m, VVP& g, QP& q) {
    vector<vector<char>> used(n, vector<char>(m));
    while(!q.empty()) {
        pair<int,int> v = q.front(); q.pop();
        int x = v.first, y = v.second;
        if (used[x][y]) continue;
        used[x][y] = 1;
        int dx[4] = {-1, 0, 1, 0};
        int dy[4] = {0, 1, 0, -1};
        for(int i = 0; i < 4; i++) {
            int nx = x + dx[i], ny = y + dy[i];
            if (nx < 0 || nx >= n || ny < 0 || ny >= m || g[nx][ny] <= g[x][y] + 1) continue;
            g[nx][ny] = 1 + g[x][y];
            q.push({nx, ny});
        }
    }
}

int main() {
    cin >> n >> m >> k;
    vector<vector<int>> g(n, vector<int>(m, inf));
    queue<pair<int, int>> q;
    for (int i = 0; i < k; i++) {
        int x, y;
        cin >> x >> y;
        q.push({x, y});
        g[x][y] = 0;
    }
    bfs(n, m, g, q);
    int ans = 0;
    for(int i = 0; i < n; i++)
        for (int j = 0; j < m; ++j)
            if(g[i][j] > ans)
                ans = g[i][j];

    cout << ans;
    return 0;
}