import java.io.*;
import java.util.*;

public class ith {
	
	
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		//BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int X,Y,N,counter = 0;
		
		
		X = sc.nextInt(); //1
		Y = sc.nextInt(); //1
		N = sc.nextInt(); //0
		
		
		int [] Nx = new int[N];
		int [] Ny = new int[N];
		
		for (int i = 0; i<N; i++) {
			Nx[ i ] = sc.nextInt()-1;
			Ny[ i ] = sc.nextInt()-1;
		}	
		
		boolean queens = false;
		
		for (int i=0; i<X; i++) {
			for (int j=0; j<Y; j++) {			
					queens = true;	
					for (int k=0; k<N; k++) {						
						if(isFree(i, j, Nx[k],Ny[k])) {
							queens=true;
						} else {
							queens=false;
							break;
						}						
					}	
					if(queens){
						counter++;
					}
			}
		}
		
		System.out.println(counter);		
	}
	
	
	public static boolean isFree(int x,int y,int dx, int dy) {
		int L = Math.abs(dx-dy);
		int P = dx+dy;
		
		if (x==dx || y==dy) return false;
		if ((x + L)==y) return false;
		if ((P - x)==y) return false;
		
		return true;
	}
	
}