#include <iostream>
#include <vector>
using namespace std;

int n, m, k, x, y, size_l, size_r;
vector<vector<bool>> obstructed;

void dfs(int i, int j, int size, vector<vector<bool>>& visited) {
	if (i < 0 || j < 0 || i + size > n || j + size > m) return;
	
	if (obstructed[i][j] || visited[i][j]) return;
	visited[i][j] = true;
	
	dfs(i - 1, j, size, visited);
	dfs(i + 1, j, size, visited);
	dfs(i, j - 1, size, visited);
	dfs(i, j + 1, size, visited);
}

bool check(int size) {
	vector<vector<bool>> visited = vector<vector<bool>>(n, vector<bool>(m, false));
	bool once = false;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < m; j++) {
			if (!obstructed[i][j] && !visited[i][j]) {
				if (once) {
					return false;
				} else {
					dfs(i, j, size, visited);
					once = true;
				}
			}
		}
	}
	
	return true;
}

int main() {
	cin >> n >> m >> k;
	obstructed = vector<vector<bool>>(n, vector<bool>(m, false));
	for (int i = 0; i < k; i++) {
		cin >> x >> y;
		obstructed[--x][--y] = true;
	}
	
	size_l = 1;
	size_r = min(n, m);
	while (size_l < size_r) {
		int size_m = (size_l + size_r) / 2;
		
		if (check(size_m)) {
		  size_l = size_m + 1;
		} else {
		  size_r = size_m;
		}
	}
	cout << size_l << endl;
}