#pragma GCC optimize(1)
#pragma GCC optimize(2)
#pragma GCC optimize(3,"Ofast","inline")
#include<bits/stdc++.h>
#define _for(i,L,R) for(int i=L;i<=R;++i)
#define build(n,m,k) vector<vector<int>>(n+5,vector<int>(m+5,k))
//#define int long long
 
using namespace std;
 
const int N=5e6+5;
int n,m,k,fa[N];

int find(int x)
{
	return x==fa[x]?x:fa[x]=find(fa[x]);
}

void merge(int x,int y)
{
	x=find(x),y=find(y);
	if(x!=y) fa[x]=y;
}

int id(int i,int j)
{
	return m*(i-1)+j;
}

vector<vector<int>> g,sumg,h,sumh;
 
void calc(int &x,int &y)
{
    x=max(1,x);y=max(1,y);
    x=min(x,n);y=min(y,m);
}
 
int getg(int x1,int y1,int x2,int y2)
{
    calc(x1,y1);calc(x2,y2);
    return sumg[x2][y2]-sumg[x1-1][y2]-sumg[x2][y1-1]+sumg[x1-1][y1-1];
}
 
int geth(int x1,int y1,int x2,int y2)
{
    calc(x1,y1);calc(x2,y2);
    return sumh[x2][y2]-sumh[x1-1][y2]-sumh[x2][y1-1]+sumh[x1-1][y1-1];
}
 
int dx[]={0,1,0,-1},dy[]={1,0,-1,0}; 
 
bool check(int mid)
{
    _for(i,1,n) _for(j,1,m){
        h[i][j]=getg(i-mid+1,j-mid+1,i,j)==mid*mid;
        sumh[i][j]=h[i][j]+sumh[i-1][j]+sumh[i][j-1]-sumh[i-1][j-1];
    }
    _for(i,1,n) _for(j,1,m) if(g[i][j] and !geth(i,j,i+mid-1,j+mid-1)) return 0;
    _for(i,1,n*m) fa[i]=i;
    _for(i,1,n) _for(j,1,m) _for(p,0,3){
    	if(!g[i][j]) continue;
    	int nx=i+dx[p],ny=j+dy[p];
        if(nx<1 or nx>n or ny<1 or ny>m or !g[nx][ny]) continue;
        merge(id(i,j),id(nx,ny));
	}
	int root=-1;
	_for(i,1,n) _for(j,1,m){
		if(!g[i][j]) continue;
		int u=find(id(i,j));
		if(root==-1) root=u;
		else if(u!=root) return 0;
	}
    return 1;
}
 
int main()
{
    scanf("%d%d%d",&n,&m,&k);
    g=build(n,m,1);
    _for(i,0,n) g[i][0]=0;
    _for(i,0,m) g[0][i]=0;
    sumg=h=sumh=build(n,m,0);
    while(k--){
        int x,y;
        scanf("%d%d",&x,&y);
        g[x][y]=0;
    }
    _for(i,1,n) _for(j,1,m) sumg[i][j]=g[i][j]+sumg[i-1][j]+sumg[i][j-1]-sumg[i-1][j-1];
    int l=0,r=min(n,m)+1;
    while(l+1<r){
        int mid=l+r>>1;
        if(check(mid)) l=mid;
        else r=mid;
    }
    if(l==0) l=-1;
    printf("%d\n",l);
    return 0;
}