#include<bits/stdc++.h>
#define ull unsigned long long
#define int long long
#define fo(i,l,r) for(int i=l;i<=r;i++)
#define fd(i,r,l) for(int i=r;i>=l;i--)
using namespace std;
int x,y,a[505],b[505],c[505],d[505],n;
bool check(int k){
	int l=-k,r=k,u=-k,D=k;
	bool flag=1;
	while(flag){
		flag=0;
		fo(i,1,n){
			if(a[i]==c[i]&&l<a[i]&&a[i]<r&&((b[i]<=u&&u<d[i])||(b[i]<D&&D<=d[i])))
				u=min(u,b[i]-1),D=max(D,d[i]+1),flag=1;
			else if(b[i]==d[i]&&u<b[i]&&b[i]<D&&((a[i]<=l&&l<c[i])||(a[i]<r&&r<=c[i])))
				l=min(l,a[i]-1),r=max(r,c[i]+1),flag=1;
		}
	}
	return x<=l||x>=r||y<=u||y>=D;
}
signed main(){
	cin>>x>>y>>n;
	fo(i,1,n){
		cin>>a[i]>>b[i]>>c[i]>>d[i];
		if(a[i]>c[i]||b[i]>d[i])swap(a[i],c[i]),swap(b[i],d[i]);
	}
	int l=0,r=LLONG_MAX,ans=0;
	while(l<=r){
		int mid=(l+r)>>1;
		if(check(mid))ans=mid,l=mid+1;
		else r=mid-1;
	}
	cout<<ans;
	return 0;
}