#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int h, w, n;
  cin >> h >> w >> n;
  static int d[2010][2010];
  rep(_, n) {
    int r1, c1, r2, c2;
    cin >> r1 >> c1 >> r2 >> c2;
    r1--, c1--;
    d[r1][c1]++;
    d[r1][c2]--;
    d[r2][c1]--;
    d[r2][c2]++;
  }
  rep(i, h) rep(j, w) d[i][j+1] += d[i][j];
  rep(i, h) rep(j, w) d[i+1][j] += d[i][j];
  int ans = 0;
  rep(i, h) rep(j, w) ans += !d[i][j];
  cout << ans << '\n';
}