/******************************************************************************

                              Online C++ Compiler.
               Code, Compile, Run and Debug C++ program online.
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*******************************************************************************/

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

int rows, columns, obstructions;
#define build(rows, columns) vector<vector<int>> (rows, vector<int>(columns, 0))
vector<vector<int>> stuff;
vector<vector<int>> visited;
vector<vector<int>> obstacles;
int currents = 0;

void dfs(int xfirst, int yfirst, int xsecond, int ysecond){
    if(xfirst < 0 || yfirst < 0 || xsecond >= rows || ysecond >= columns || visited[xfirst][yfirst] == 1){
        return;
    }
    int howmany = 0;
    if (xfirst > 0 && yfirst > 0){
        howmany = stuff[xsecond][ysecond] - stuff[xfirst-1][ysecond] - stuff[xsecond][yfirst-1] + stuff[xfirst-1][yfirst-1];
    }else if(xfirst == 0 && yfirst == 0){
        howmany = stuff[xsecond][ysecond];
    }else if(xfirst > 0){
        howmany = stuff[xsecond][ysecond] - stuff[xfirst-1][ysecond];
    }else if(yfirst > 0){
        howmany = stuff[xsecond][ysecond] - stuff[xsecond][yfirst-1];
    }
    if (howmany > 0){
        return;
    }
    visited[xfirst][yfirst] = 1;
    dfs(xfirst-1, yfirst, xsecond-1, ysecond);
    dfs(xfirst+1, yfirst, xsecond+1, ysecond);
    dfs(xfirst, yfirst-1, xsecond, ysecond-1);
    dfs(xfirst, yfirst+1, xsecond, ysecond+1);
}

int checks(int which){
    visited = build(rows, columns);
    int checkers = 0;
    for (int a = 0; a <= rows-which; ++a){
        for (int b = 0; b <= columns-which; ++b){
            int xfirst = a;
            int yfirst = b;
            int xsecond = a+which-1;
            int ysecond = b+which-1;
            int howmany = 0;
            if (xfirst > 0 && yfirst > 0){
                howmany = stuff[xsecond][ysecond] - stuff[xfirst-1][ysecond] - stuff[xsecond][yfirst-1] + stuff[xfirst-1][yfirst-1];
            }else if(xfirst == 0 && yfirst == 0){
                howmany = stuff[xsecond][ysecond];
            }else if(xfirst > 0){
                howmany = stuff[xsecond][ysecond] - stuff[xfirst-1][ysecond];
            }else if(yfirst > 0){
                howmany = stuff[xsecond][ysecond] - stuff[xsecond][yfirst-1];
            }
            if (howmany == 0){
                dfs(xfirst, yfirst, xsecond, ysecond);
                checkers = 1;
                break;
            }
        }
        if (checkers == 1){
            break;
        }
    }
    if (checkers == 0){
        return 0;
    }
    checkers = 1;
    for (int c = 0; c < rows; ++c){
        for (int d = 0; d < columns; ++d){
            if (visited[c][d] != 1 && obstacles[c][d] != 1){
                checkers = 0;
                break;
            }
        }
        if (checkers == 0){
            break;
        }
    }
    return checkers;
}

int main(){
    cin >> rows >> columns >> obstructions;
    stuff = build(rows, columns);
    visited = build(rows, columns);
    obstacles = build(rows, columns);
    for (int a = 0; a < obstructions; ++a){
        int xpos, ypos;
        cin >> xpos >> ypos;
        xpos -= 1;
        ypos -= 1;
        stuff[xpos][ypos] = 1;
        obstacles[xpos][ypos] = 1;
    }
    for (int a = 0; a < rows; ++a){
        for (int b = 0; b < columns; ++b){
            if (a == 0 || b == 0){
                if (a == 0 && b == 0){
                    //
                }else if(a == 0){
                    stuff[a][b] += stuff[a][b-1];
                }else{
                    stuff[a][b] += stuff[a-1][b];
                }
            }else{
                stuff[a][b] += stuff[a-1][b];
                stuff[a][b] += stuff[a][b-1];
                stuff[a][b] -= stuff[a-1][b-1];
            }
        }
    }
    int l = 0;
    int r = min(rows, columns)+1;
    while (l<r){
        //cout << l << " " << r << endl;
        int mids = (l+r)/2;
        if (checks(mids) == 1){
            l = mids+1;
        }else{
            r = mids;
        }
    }
    if (l == 0){
        cout << -1;
    }else{
        cout << l;
    }
}