/* -*- coding: utf-8 -*-
 *
 * 3709.cc:  No.3709 Unknown Treasure - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_H = 2000;
const int MAX_W = 2000;

/* typedef */

/* global variables */

int cs[MAX_H + 1][MAX_W + 1];
int css[MAX_H + 1][MAX_W + 1];

/* subroutines */

/* main */

int main() {
  int h, w, n;
  scanf("%d%d%d", &h, &w, &n);
  for (int i = 0; i < n; i++) {
    int r0, c0, r1, c1;
    scanf("%d%d%d%d", &r0, &c0, &r1, &c1);
    r0--, c0--;
    cs[r0][c0]++, cs[r1][c0]--, cs[r0][c1]--, cs[r1][c1]++;
  }

  for (int i = 0; i < h; i++)
    for (int j = 0; j < w; j++)
      css[i + 1][j + 1] =
	cs[i][j] + css[i + 1][j] + css[i][j + 1] - css[i][j];

  int cnt = 0;
  for (int i = 1; i <= h; i++)
    for (int j = 1; j <= w; j++)
      if (css[i][j] == 0) cnt++;

  printf("%d\n", cnt);
  
  return 0;
}