import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		int h = Integer.parseInt(sa[0]);
		int w = Integer.parseInt(sa[1]);
		int n = Integer.parseInt(sa[2]);
		int[][] a = new int[h + 1][w + 1];
		for (int i = 0; i < n; i++) {
			sa = br.readLine().split(" ");
			int r1 = Integer.parseInt(sa[0]) - 1;
			int c1 = Integer.parseInt(sa[1]) - 1;
			int r2 = Integer.parseInt(sa[2]);
			int c2 = Integer.parseInt(sa[3]);
			a[r1][c1]++;
			a[r1][c2]--;
			a[r2][c1]--;
			a[r2][c2]++;
		}
		br.close();

		for (int i = 0; i < h; i++) {
			for (int j = 1; j <= w; j++) {
				a[i][j] += a[i][j - 1];
			}
		}
		for (int i = 1; i <= h; i++) {
			for (int j = 0; j < w; j++) {
				a[i][j] += a[i - 1][j];
			}
		}

		int ans = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (a[i][j] == 0) {
					ans++;
				}
			}
		}
		System.out.println(ans);
	}
}