#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using pint=pair<int,int>;
#define rep(i,n) for(int i=0;i<(n);++i)
template<typename T>
void chmax(T& a,T b){if(a<b){a=b;};}
template<typename T>
void chmin(T& a,T b){if(a>b){a=b;};}
const int inf=1e9;

int main(){
  int h,w,n;cin>>h>>w>>n;
  vector sum(h+1,vector<int>(w+1));
  rep(i,n){
    int r1,c1,r2,c2;cin>>r1>>c1>>r2>>c2;
    r1--,c1--;
    sum[r1][c1]++;
    sum[r1][c2]--;
    sum[r2][c1]--;
    sum[r2][c2]++;
  }
  rep(i,h+1)rep(j,w)sum[i][j+1]+=sum[i][j];
  rep(i,w+1)rep(j,h)sum[j+1][i]+=sum[j][i];
  int ans=0;
  rep(i,h)rep(j,w)ans+=sum[i][j]==0;
  cout<<ans<<endl;
}