/*
点(0,0)对简单多边形的张角
*/ 
#include<stdio.h>
#include<string.h>
#include<math.h>
#define pi acos(-1.0)
struct Nodes{
	double x,y;
}p[110];
double k,h;
int n;
double get_angle(Nodes a,Nodes b)/*-pi<angle<pi*/
{
	double angle1,angle2;
	angle1=atan2(a.y,a.x);
	angle2=atan2(b.y,b.x);
	if(angle1-angle2>pi)
		angle2+=2*pi;
	if(angle2-angle1>pi)
		angle1+=2*pi;
	return angle1-angle2;
}
int main()
{
	int i,j;
	double maxx,minn,sum;
	scanf("%lf%lf%d",&k,&h,&n);
		for(i=0;i<n;i++){
			scanf("%lf%lf",&p[i].x,&p[i].y);
		}
		p[n]=p[0];
		maxx=minn=sum=0;
		for(i=0;i<n;i++){
			sum+=get_angle(p[i],p[i+1]);
			if(sum>maxx)
				maxx=sum;
			if(sum<minn)
				minn=sum;
			if(maxx-minn>2*pi){
				maxx=minn+2*pi;
				break;
			}
		}
		printf("%.2lf\n",k*h*(maxx-minn));/*angle=maxx-minn*/
	return 0;
}