#include <iostream>
#include <vector>
#include <string>
using namespace std ;
vector<vector<int>> ah, av, seen, as ;
vector<int> row, cnts ;
int cookie ;
vector<string> b ;
int main(int argc, char *argv[]) {
   int w, h, m ;
   cin >> h >> w >> m ;
   b.resize(h) ;
   ah.resize(h+1) ;
   av.resize(h+1) ;
   seen.resize(h) ;
   as.resize(h+1) ;
   row.resize(w+1) ;
   cnts.resize(h+1) ;
   for (int i=0; i<h; i++) {
      b[i].resize(w, '.') ;
      ah[i].resize(w+1) ;
      av[i].resize(w+1) ;
      seen[i].resize(w) ;
      as[i].resize(w+1) ;
   }
   ah[h].resize(w+1) ;
   av[h].resize(w+1) ;
   as[h].resize(w+1) ;
   for (int i=0; i<m; i++) {
      int x, y ;
      cin >> x >> y ;
      b[x-1][y-1] = '#' ;
   }
   for (int i=h-1; i>=0; i--)
      for (int j=w-1; j>=0; j--)
         if (b[i][j] == '.')
            ah[i][j] = 1 + ah[i][j+1] ;
   for (int j=w-1; j>=0; j--)
      for (int i=h-1; i>=0; i--)
         if (b[i][j] == '.')
            av[i][j] = 1 + av[i+1][j] ;
   int hi = 0 ;
   for (int i=h-1; i>=0; i--) {
      for (int j=w-1; j>=0; j--) {
         as[i][j] = min(as[i+1][j+1]+1, min(ah[i][j], av[i][j])) ;
         hi = max(hi, as[i][j]) ;
         cnts[as[i][j]]++ ;
      }
   }
   for (int i=hi-1; i>=0; i--)
      cnts[i] += cnts[i+1] ;
   int lo = 0 ;
   int bit = 1 ;
   while (bit <= hi)
      bit <<= 1 ;
   for (; bit; bit>>=1) {
      int t = bit + lo ;
      if (t > hi)
         continue ;
      cookie++ ;
      vector<pair<int, int>> q ;
      for (int i=0; q.size() == 0; i++)
         for (int j=0; q.size() == 0 && j<w; j++)
            if (as[i][j] >= t)
               q.push_back({i,j}) ;
      seen[q[0].first][q[0].second] = cookie ;
      for (int qg=0; qg<(int)q.size(); qg++) {
         auto [i,j] = q[qg] ;
         if (i>0 && as[i-1][j] >= t && seen[i-1][j] != cookie) {
            q.push_back({i-1,j}) ;
            seen[i-1][j] = cookie ;
         }
         if (i+1<h && as[i+1][j] >= t && seen[i+1][j] != cookie) {
            q.push_back({i+1,j}) ;
            seen[i+1][j] = cookie ;
         }
         if (j>0 && as[i][j-1] >= t && seen[i][j-1] != cookie) {
            q.push_back({i,j-1}) ;
            seen[i][j-1] = cookie ;
         }
         if (j+1<w && as[i][j+1] >= t && seen[i][j+1] != cookie) {
            q.push_back({i,j+1}) ;
            seen[i][j+1] = cookie ;
         }
      }
      if (cnts[t] != (int)q.size())
         continue ;
      for (int j=0; j<w; j++)
         row[j] = 0 ;
      int good = 1 ;
      for (int i=0; good && i<h; i++) {
         int sf = 0 ;
         for (int j=0; j<w; j++)
            row[j] = max(0, row[j]-1) ;
         for (int j=0; j<w; j++) {
            if (as[i][j] >= t) {
               sf = max(j, sf) ;
               while (sf < j+t)
                  row[sf++] = t ;
            }
         }
         for (int j=0; good==1 && j<w; j++)
            if (b[i][j] == '.' && row[j] == 0)
               good = 0 ;
      }
      if (good)
         lo = t ;
   }
   if (lo == 0)
      lo = -1 ;
   cout << lo << endl ;
}