#include <iostream>
#include <vector>
#include <map>
#include <limits.h>
#include <algorithm>
using namespace std;

map<vector<int>, int> visited;
int n, m, k;

map<vector<int>, int> obs;


void dfs(int x, int y){
  //vector<int> loc = {xpos, ypos, zpos};
  if (visited[{x, y}] == 1) {
    return;
  }
  /*visited.push_back(loc);
  for (vector<int> neighbor : adj_list[loc]) {

    dfs(neighbor[0], neighbor[1], neighbor[2]);
  }*/
  visited[{x, y}] = 1;

  if (x+1 <= n && obs[{x+1, y}] != 1) {
    dfs(x+1, y);
  }
  if (x-1 > 0 && obs[{x-1, y}] != 1) {
    dfs(x-1, y);
  }

  if (y+1 <= m && obs[{x, y+1}] != 1) {
    dfs(x, y+1);
  }
  if (y-1 > 0 && obs[{x, y-1}] != 1) {
    dfs(x, y-1);
  }
}

int main() {
  //std::cout << "Hello World!\n";
  
  
  cin >> n >> m >> k;
  if (k == m*n) {
    //cout << "one\n";
    cout << -1 << "\n";
  } else {
    map<int, vector<int>> xs;
    map<int, vector<int>> ys;

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

      if (xs.find(x) == xs.end()) {
        xs[x] = {y};
      } else {
        xs[x].push_back(y);
      }

      if (ys.find(y) == ys.end()) {
        ys[y] = {x};
      } else {
        ys[y].push_back(x);
      }

      obs[{x, y}] = 1;

    }

    int firstx = 1;
    int firsty = 1;
    bool found = 0;

    for(int i = 1; i < n+1; i++) {
      for(int j = 1; j < m+1; j++) {
        if (obs.count({i, j})) {
          firstx = 1;
          firsty = 1;
          found = 1;
          break;
        }
        if (found) {
          break;
        }
      }
    }

    dfs(firstx, firsty);
    if (visited.size() < n*m - k) {
      //cout << "two\n";
      cout << -1 << "\n";
    } else {
      int maxside;
      //if (k == 0) {
      maxside = min(n, m);
      /*} else {
        maxside = min({n - maxx, m - maxy, minx - 1, miny - 1});
      }*/

      int minx = -1;
      int miny = -1;
      int maxx = 0;
      int maxy = 0;


      map<int, vector<int>> ::iterator itx;
      for (itx = xs.begin(); itx != xs.end(); itx++)
      {
        vector<int> vals = itx->second;
        sort(vals.begin(), vals.end());
        for (int j = 0; j < vals.size() - 1; j++) {
          if (vals[j+1] - vals[j] - 1 != 0 && vals[j+1] - vals[j] - 1 < maxside) {
            maxside = vals[j+1] - vals[j] - 1;
          }
          if (miny == -1 && vals[j] != 1) {
            miny = vals[j];
          }
          if (vals[j] != m) {
            maxy = vals[j];
          }
        }

        if (miny == -1 && vals[vals.size()-1] != 1) {
          miny = vals[vals.size()-1];
        }
        if (vals[vals.size()-1] != m) {
          maxy = vals[vals.size()-1];
        }
      }

      map<int, vector<int>> ::iterator ity;
      for (ity = ys.begin(); ity != ys.end(); ity++)
      {
        vector<int> vals = ity->second;
        sort(vals.begin(), vals.end());
        for (int j = 0; j < vals.size() - 1; j++) {
          if (vals[j+1] - vals[j] - 1 != 0 && vals[j+1] - vals[j] - 1 < maxside) {
            maxside = vals[j+1] - vals[j] - 1;
          }
          if (minx == -1 && vals[j] != 1) {
            minx = vals[j];
          }
          if (vals[j] != n) {
            maxx = vals[j];
          }
        }
        if (minx == -1 && vals[vals.size()-1] != 1) {
          minx = vals[vals.size()-1];
        }
        if (vals[vals.size()-1] != n) {
          maxx = vals[vals.size()-1];
        }
      }
      //cout << "maxside: " << maxside << "\n";
      maxside = min({maxside, n - maxx, m - maxy});
      //cout << "maxside: " << maxside << "\n";

      if (minx != -1) {
        maxside = min(maxside, minx-1);
        //cout << "xmaxside: " << maxside << "\n";
      }
      if (miny != -1) {
        maxside = min(maxside, miny-1);
        //cout << "ymaxside: " << maxside << "\n";
      }
      
      //cout << "three\n";
      cout << maxside << "\n";
    }
  }
}