#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5e6 + 5;
typedef long long ll;

int n, m, k;
vector <int> mp[MAXN], cnt[MAXN];
vector <bool> blk[MAXN], vis[MAXN];

void dfs(int x, int y) {
	vis[x][y] = false;
	if (vis[x + 1][y]) dfs(x + 1, y);
	if (vis[x][y + 1]) dfs(x, y + 1);
	if (vis[x - 1][y]) dfs(x - 1, y);
	if (vis[x][y - 1]) dfs(x, y - 1);
}
bool check(int mid) {
	for (int i = 0; i <= n + 1; i++)
	for (int j = 0; j <= m + 1; j++)
		cnt[i][j] = vis[i][j] = 0;

	int x = 1, y = 1;
	for (int i = mid; i <= n; i++)
	for (int j = mid; j <= m; j++)
		if (mp[i][j] - mp[i - mid][j] - mp[i][j - mid] + mp[i - mid][j - mid] == mid * mid) {
			cnt[i][j] += 1;
			cnt[i - mid][j] -= 1;
			cnt[i][j - mid] -= 1;
			cnt[i - mid][j - mid] += 1;
			vis[i][j] = true;
			x = i, y = j;
		}
	for (int i = n; i >= 1; i--)
	for (int j = m; j >= 1; j--) {
		cnt[i][j] += cnt[i + 1][j] + cnt[i][j + 1] - cnt[i + 1][j + 1];
		if (cnt[i][j] == 0 && !blk[i][j]) return false;
	}
	dfs(x, y);
	for (int i = mid; i <= n; i++)
	for (int j = mid; j <= m; j++)
		if (vis[i][j]) return false;
	return true;
}

int main() {
	scanf("%d%d%d", &n, &m, &k);
	for (int i = 0; i <= n + 1; i++) {
		mp[i].resize(m + 2);
		cnt[i].resize(m + 2);
		vis[i].resize(m + 2);
		blk[i].resize(m + 2);
	}
	for (int i = 1; i <= n; i++)
	for (int j = 1; j <= m; j++)
		mp[i][j] = 1;
	for (int i = 1; i <= k; i++) {
		int x, y; scanf("%d%d", &x, &y);
		mp[x][y] = 0; blk[x][y] = true;
	}
	for (int i = 1; i <= n; i++)
	for (int j = 1; j <= m; j++)
		mp[i][j] += mp[i - 1][j] + mp[i][j - 1] - mp[i - 1][j - 1];
	int l = 0, r = min(n, m);
	while (l < r) {
		int mid = (l + r + 1) / 2;
		if (check(mid)) l = mid;
		else r = mid - 1;
	}
	if (l == 0) cout << -1 << endl;
	else cout << l << endl;
	return 0;
}