#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
mt19937 mrand(random_device{}()); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head

int dx[]={0,1,0,-1},dy[]={-1,0,1,0};
int n,m,k;
int main() {
	scanf("%d%d%d",&n,&m,&k);
	vector<vector<int>> ob(n+1,VI(m+1,0));
	vector<vector<int>> ob2(n,VI(m,0));
	rep(i,0,k) {
		int u,v;
		scanf("%d%d",&u,&v);
		ob[u][v]=1;
		ob2[u-1][v-1]=1;
	}
	rep(i,1,n+1) rep(j,1,m+1) ob[i][j]+=ob[i][j-1];
	rep(i,1,n+1) rep(j,1,m+1) ob[i][j]+=ob[i-1][j];
	PII fp(-1,-1);
	per(i,0,n) per(j,0,m) if (ob2[i][j]==0) fp=mp(i,j);
	int l=0,r=min(n,m)+1;
	auto check=[&](int x) {
		vector<vector<int>> pos(n,VI(m,0));
		vector<vector<int>> vis(n,VI(m,0));
		vector<vector<int>> mark(n+1,VI(m+1,0));
		rep(i,0,n) rep(j,0,m) if (i+x<=n&&j+x<=m) {
			int v=ob[i+x][j+x]-ob[i+x][j]-ob[i][j+x]+ob[i][j];
			if (v==0) {
				pos[i][j]=1;
			}
		}
		if (pos[fp.fi][fp.se]==0) return false;
		queue<PII> q;
		vis[fp.fi][fp.se]=1;
		q.push(fp);
		while (!q.empty()) {
			auto [x, y]=q.front();
			q.pop();
			rep(d,0,4) if (x+dx[d]>=0&&x+dx[d]<n&&y+dy[d]>=0&&y+dy[d]<m) {
				int nx=x+dx[d],ny=y+dy[d];
				if (pos[nx][ny]==1&&!vis[nx][ny]) {
					vis[nx][ny]=1;
					q.push({nx,ny});
				}
			}
		}
		rep(i,0,n) rep(j,0,m) if (vis[i][j]) {
			mark[i][j]++; mark[i+x][j+x]++; mark[i][j+x]--; mark[i+x][j]--;
		}
		rep(i,0,n) rep(j,1,m) mark[i][j]+=mark[i][j-1];
		rep(i,1,n) rep(j,0,m) mark[i][j]+=mark[i-1][j];
		rep(i,0,n) rep(j,0,m) if (ob2[i][j]==0&&mark[i][j]==0) return false;
		return true;
	};
	while (l+1<r) {
		int md=(l+r)>>1;
		if (check(md)) l=md; else r=md;
	}
	if (l==0) l=-1;
	printf("%d\n",l);
}