#include <bits/stdc++.h>
using namespace std;

int n, m, k;
vector<vector<int>> grid;
vector<vector<int>> maze;

bool bfs(int x, int y) {
  int dx[4] = {0, 0, 1, -1};
  int dy[4] = {1, -1, 0, 0};

  int vis = 1;
  vector<vector<bool>> visited(n + 1, vector<bool>(m + 1, false));
  queue<pair<int, int>> q;

  q.push({x, y});
  visited[x][y] = true;

  while (!q.empty()) {
    pair<int, int> curr = q.front();
    q.pop();

    for (int i = 0; i < 4; i++) {
      int nx = curr.first + dx[i];
      int ny = curr.second + dy[i];

      if (nx >= 1 && nx <= n && ny >= 1 && ny <= m && !visited[nx][ny] && maze[nx][ny] == 0) {
        visited[nx][ny] = true;
        q.push({nx, ny});
        vis++;
      }
    }
  }

  return vis == (n*m) - k;
}

bool valid(int x) {
  vector<vector<int>> clean(n + 5, vector<int>(m + 5, 0));
  for (int i = x; i <= n; i++) {
    for (int j = x; j <= m; j++) {
      int cnt = grid[i][j] - grid[i - x][j] - grid[i][j - x] + grid[i - x][j - x];
      if (cnt == 0) {
        clean[i - x + 1][j - x + 1]++;
        clean[i - x + 1][j + 1]--;
        clean[i + 1][j - x + 1]--;
        clean[i + 1][j + 1]++;
      }
    }
  }

  int vis = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      clean[i][j] += clean[i - 1][j] + clean[i][j - 1] - clean[i - 1][j - 1];
      if (clean[i][j]) vis++;
    }
  }

  return vis == (n*m) - k;
}

int main() {
  cin >> n >> m >> k;

  grid.resize(n + 5, vector<int>(m + 5, 0));
  maze.resize(n + 5, vector<int>(m + 5, 0));

  for (int i = 0; i < k; i++) {
    int x, y;
    cin >> x >> y;

    grid[x][y] = 1;
    maze[x][y] = 1;
  }

  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      grid[i][j] += grid[i - 1][j] + grid[i][j - 1] - grid[i - 1][j - 1];
    }
  }
  
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      if (maze[i][j] == 0) {
        if (!bfs(i, j)) {
          cout << -1 << "\n";
          return 0;
        }
        else {
          break;
        }
      }
    }
  }

  int l = 1, r = min(n,m);
  while (l <= r) {
    int mid = (l + r) / 2;
    if (valid(mid)) {
      l = mid + 1;
    }
    else {
      r = mid - 1;
    }
  }

  cout << r << "\n";
  return 0;
}