import java.util.*; import java.awt.Point;

//thank me later public class Solution { public static void main(String[] args){ Scanner in = new Scanner(System.in);

    int n = in.nextInt();
    int k = in.nextInt();

    Point queen = new Point(in.nextInt(), in.nextInt());

    HashSet<Point> obstacles = new HashSet<Point>();

    for(int i = 0; i < k; i++){
        obstacles.add(new Point(in.nextInt(), in.nextInt()));
    }

    Stack<Direction> directions = getAllDirections();
    int possiblePoints = 0;

    while(!directions.empty()){
        Direction d = directions.pop();
        Point cur = new Point(queen);
        cur.translate(d.x, d.y);

        while(isInside(cur, n) && !obstacles.contains(cur)){
            possiblePoints++;
            cur.translate(d.x, d.y);
        }
    }

    System.out.println(possiblePoints);
}

private static boolean isInside(Point p, int n){
    return p.x > 0 && p.y > 0 && p.x <= n && p.y <= n;
}

private static Stack<Direction> getAllDirections(){
    Stack<Direction> directions = new Stack<Direction>();
    directions.push(new Direction(0, 1));
    directions.push(new Direction(1, 1));
    directions.push(new Direction(1, 0));
    directions.push(new Direction(1, -1));
    directions.push(new Direction(0, -1));
    directions.push(new Direction(-1, -1));
    directions.push(new Direction(-1, 0));
    directions.push(new Direction(-1, 1));
    return directions;
}

private static class Direction {
    int x, y;
    public Direction(int x, int y){
        this.x = x;
        this.y = y;
    }
}

}