//2022.1.28 by ljz
//email [email protected]
//if you find any bug in my code
//please tell me
#include<bits/stdc++.h>
//#include<ext/pb_ds/tree_policy.hpp>
//#include<ext/pb_ds/assoc_container.hpp>
using namespace std;
//using namespace __gnu_pbds;
//using namespace __gnu_cxx;
#define res int
#define LL long long
#define Inf 0x3f3f3f3f
#define sup 0x7fffffff
#define inf 0x3f3f3f3f
#define INF 2000000000000000000
//#define unl __int128
#define eps 1e-10
#define RG
#define db double
#define pc(x) __builtin_popcount(x)
#define ctz(x) __builtin_ctz(x)
//#define pc(x) __builtin_popcountll(x)
typedef pair<int,int> Pair;
//#define poly vector<int>
#define mp make_pair
#define fi first
#define se second
#define pi acos(-1.0)
#define pb push_back
#define ull unsigned LL
#define uint unsigned int
#define lowbit(x) ((x)&-(x))
#define gc getchar
#define ld long db
//template <class T>using Tree=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
//inline int gc() {
//    static char buf[100000],*p1,*p2;
//    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
//}
//char sr[1<<21],z[20];
//int C=-1,Z=0;
//inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}
//inline void print(RG LL x){
//    if(C>1<<20)Ot();if(x<0)sr[++C]='-',x=-x;
//    while(z[++Z]=x%10+48,x/=10);
//    while(sr[++C]=z[Z],--Z);
//}
template <typename T> inline void Read(T &x) {
    res c=gc();
    bool f=false;
    for(x=0;!isdigit(c);c=gc())if(c=='-')f=true;
    for(;isdigit(c);c=gc())x=x*10+c-'0';
    if(f)x=-x;
}
inline int read() {
    res s=0,ch=gc(),w=1;
    while(ch<'0'||ch>'9'){
        if(ch=='-')w=-1;
        else if(ch==EOF)break;
        ch=gc();
    }
    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
    return s*w;
}
inline LL Read() {
    RG LL s=0;
    res ch=gc(),w=1;
    while(ch<'0'||ch>'9'){
        if(ch=='-')w=-1;
        else if(ch==EOF)break;
        ch=gc();
    }
    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
    return s*w;
}
inline void write(RG __int128 x){
    if(x>10)write(x/10);
    putchar(int(x%10)+'0');
}
const int kcz=998244353;
const int G=3,GI=332748118;
//inline void add(res &x,const res &y){
//    x+=y,x>=kcz?x-=kcz:1;
//}
//inline int Add(const res &x,const res &y){
//    return x+y>=kcz?x+y-kcz:x+y;
//}
//inline int mul(const res &x,const res &y){
//    return int(1ll*x*y%kcz);
//}
#define add(x,y) ((x)+=(y),(x)>=kcz?(x)-=kcz:1)
#define Add(x,y) ((x)+(y)>=kcz?(x)+(y)-kcz:(x)+(y))
#define mul(x,y) (int)((LL)(x)*(y)%kcz)
#define Mul(x,y,d) (int)((ull)(x)*(y)/(d)%kcz)
inline int qpow(res x,res y=kcz-2){
    res ret=1;
    while(y){
        if(y&1)ret=mul(ret,x);
        x=mul(x,x),y>>=1;
    }
    return ret;
}
inline int qpow(res x,res y,const res &ljc){
    res ret=1;
    while(y){
        if(y&1)ret=(int)(1ll*ret*x%ljc);
        x=(int)(1ll*x*x%ljc),y>>=1;
    }
    return ret;
}
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
//cloclim_t start=cloclim();
//inline void clim(){
//    if(1.0*(cloclim()-start)/CLOCKS_PER_SEC>0.1)exit(0);
//}
//2022.1.28 by ljz
//email [email protected]
//if you find any bug in my code
//please tell me
const int N=5e6+10;
const int dx[]={0,0,1,-1},dy[]={1,-1,0,0};
namespace MAIN{
    int n,m,k;
    vector<int> A[N],B[N],C[N];
    Pair Q[N];
    int he,ta;
    int vis[N];
    inline int bfs(const res &si,const res &sj){
        Q[he=ta=1]=mp(si,sj),vis[(si-1)*m+sj]=1;
        while(he<=ta){
            auto [x,y]=Q[he++];
            for(res l=0;l<4;l++){
                res nx=x+dx[l],ny=y+dy[l];
                if(nx<1||ny<1||nx>n||ny>m)continue;
                if(C[nx][ny])continue;
                if(vis[(nx-1)*m+ny])continue;
                Q[++ta]=mp(nx,ny),vis[(nx-1)*m+ny]=1;
            }
        }
        return ta;
    }
    inline void MAIN(){
        n=read(),m=read(),k=read();
        for(res i=0;i<=n+1;i++)A[i].resize(m+2),C[i].resize(m+2);
        for(res i=1;i<=k;i++){
            res x=read(),y=read();
            A[x][y]=C[x][y]=1;
        }
        for(res i=1;i<=n;i++){
            for(res j=1;j<=m;j++){
                if(!C[i][j]){
                    if(bfs(i,j)!=n*m-k){puts("-1");return;}
                    break;
                }
            }
        }
        for(res i=1;i<=n;i++)for(res j=1;j<=m;j++){
            A[i][j]+=A[i-1][j]+A[i][j-1]-A[i-1][j-1];
        }
        res l=1,r=min(n,m),ret=0;
        while(l<=r){
            res mid=(l+r)>>1;
            for(res i=0;i<=n+1;i++)vector<int>(m+2).swap(B[i]);
            for(res i=1;i<=n;i++){
                for(res j=1;j<=m;j++){
                    if(i+mid-1<=n&&j+mid-1<=m){
                        res t=A[i+mid-1][j+mid-1]-A[i+mid-1][j-1]-A[i-1][j+mid-1]+A[i-1][j-1];
//                        if(mid==3)printf("%d %d %d\n",i,j,t);
                        if(!t)B[i][j]++,B[i+mid][j]--,B[i][j+mid]--,B[i+mid][j+mid]++;
                    }
                }
            }
            for(res i=1;i<=n;i++)for(res j=1;j<=m;j++)B[i][j]+=B[i-1][j]+B[i][j-1]-B[i-1][j-1];
            res fl=0;
            for(res i=1;i<=n;i++){
                for(res j=1;j<=m;j++){
                    if(C[i][j])continue;
//                    if(mid==3)printf("%d %d %d\n",i,j,B[i][j]);
                    if(!B[i][j]){fl=1;break;}
                }
                if(fl)break;
            }
            if(fl)r=mid-1;
            else l=mid+1,ret=mid;
        }
        printf("%d\n",ret);
    }
}
int main(){
//    srand(time(0));
//    freopen("1.in","r",stdin);
//    freopen("1.out","w",stdout);
    res Case=1;
    for(res T=1;T<=Case;T++)MAIN::MAIN();
//    Ot();
    return 0;
}