//...
#include<iostream>
#include<stdio.h>
#include<math.h>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<map>
#include<queue>
#include<string>
#define Pi 3.14159265358
#define mod9 %1000000009
#define mod7 %1000000007
#define INF 1000000000
#define LL  long long
using namespace std;
 int n,m,k,i,j,x,y,xa,ya,xb,yb;
 LL ans;
 bool a[6002][6004];
 LL d[6002],h[6002];
 vector<pair<int,int> > p;
int main(){
    scanf("%d%d%d",&m,&n,&k);
    for (i=1;i<=m;i++)
        for (j=1;j<=n;j++)
            a[i][j]=1;
    for (i=1;i<=k;i++){
        scanf("%d%d%d%d",&xa,&ya,&xb,&yb);
        if (xa>xb) swap(xa,xb);
        if (ya>yb)swap(ya,yb);
        xa++;ya++;
        for (x=xa-1;x<=xb+1;x++)
            for (y=ya-1;y<=yb+1;y++){
                a[x][y]=0;
            }
        ans=max(ans,(xb-xa+1)*1ll*(yb-ya+1));
    }

    for (i=1;i<=m;i++){
        p.clear();
        for (j=1;j<=n;j++){
            if (a[i][j]==0) h[j]=0; else h[j]=h[j]+1;
            while (p.size() && p[p.size()-1].first>=h[j]) p.pop_back();
            if (p.size()==0) d[j]=h[j]*j;
            else {
                x=p[p.size()-1].second;
                y=p[p.size()-1].first;
                d[j]=max(h[j]*(j-x),d[x]+y*(j-x));
            }
            ans=max(ans,max(d[j],h[j]));
            p.push_back(make_pair(h[j],j));
        };
    }
    cout<<ans<<endl;
 return 0;
}