#pragma GCC optimize(1)
#pragma GCC optimize(2)
#pragma GCC optimize(3,"Ofast","inline")
#include<iostream>
#include<algorithm>
#include<vector>
#include<queue> 
#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;

using node=pair<int,int>;
#define x first
#define y second
int n,m,k;
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};

void bfs(int sx,int sy)
{
	queue<node> que;
	que.push({sx,sy});
	while(que.size()){
		int x=que.front().x,y=que.front().y;
		que.pop();
		h[x][y]=0;
		_for(i,0,3){
			int nx=x+dx[i],ny=y+dy[i];
			if(nx<1 or nx>n or ny<1 or ny>m or !h[nx][ny]) continue;
			que.push({nx,ny});
		}
	}
}


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;
	int ok=0;
	_for(i,1,n){
		_for(j,1,m) if(h[i][j]){
			bfs(i,j);
			ok=1;
			break;
		}
		if(ok) break;
	}
	_for(i,1,n) _for(j,1,m) if(h[i][j]) 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;
}