/*
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 * and open the template in the editor.
 */

/**
 *
 * @author cteam32
 */
import java.io.IOException;

/**
 * @author cteam32
 */
public class ith {

    public static void main(String[] args) throws IOException {

        String input = nextLine();
        String[] abn = input.split(" ");



        int a = Integer.valueOf(abn[0]);
        int b = Integer.valueOf(abn[1]);
        int n = abn.length == 3 ? Integer.valueOf(abn[2]) : 0;

        if (n == 0) {
            System.out.print(a * b);
        } else {
            boolean[][] pole = new boolean[a][b];

            int i;
            int j;
            for (i = 0; i < pole.length; i++) {
                for (j = 0; j < pole[0].length; j++) {
                    pole[i][j] = true;
                }
            }

            byte max = (byte) (a > b ? b : a);

            String[] abn2;
            for (int t = 0; t <= n; t++) {
                String input2 = nextLine();
                abn2 = input2.split(" ");
                int x = Integer.valueOf(abn2[0]) - 1;
                int y = Integer.valueOf(abn2[1]) - 1;

                if (x > 0 && y > 0) {


                    pole[x][y] = false;

                    int k;
                    for (k = 0; k < a; k++) {
                        pole[k][y] = false;
                    }
                    for (k = 0; k < b; k++) {
                        pole[x][k] = false;
                    }

                    for (k = -max; k < max; k++) {
                        if (x + k < a && y + k < b && x + k > -1 && y + k > -1) {
                            pole[x + k][y + k] = false;
                        }
                        if (x - k < a && y + k < b && x - k > -1 && y + k > -1) {
                            pole[x - k][y + k] = false;
                        }
                        if (x + k < a && y - k < b && x + k > -1 && y - k > -1) {
                            pole[x + k][y - k] = false;
                        }
                        if (x - k < a && y - k < b && x - k > -1 && y - k > -1) {
                            pole[x - k][y - k] = false;
                        }
                    }
                }
            }


            int pocet = 0;
            for (i = 0; i < pole.length; i++) {
                for (j = 0; j < pole[0].length; j++) {
                    if (pole[i][j]) {
                        pocet++;
                    }
                }
            }
            System.out.print(pocet);
        }
    }

    public static String nextLine() throws IOException {
        int z;
        String ret = "";
        while ((z = ((char) (System.in.read()))) != '\n') {
            ret += (char) z;
        }
        return ret;
    }
}