#include <stdio.h>
#include <math.h>

#define M_PI 3.14159265358979323846

double k,h,x[101],y[101];

double angle(double x0, double y0, double x1, double y1) {
     double a=atan2(y0, x0);
     double b=atan2(y1, x1);
     if(b-a>M_PI) a+=2*M_PI;
     if(a-b>M_PI) b+=2*M_PI;
     return a-b;
}

int main(void) {
     int i,n;
     scanf("%lf %lf %d",&k,&h,&n);
     for(i=0;i<n;i++)
          scanf("%lf %lf",&x[i],&y[i]);
     x[n]=x[0],y[n]=y[0];

     double min=0,max=0,sum=0;
     for(i = 0; i < n; i++) {
         double temp=angle(x[i],y[i],x[i + 1],y[i + 1]);
                  sum+=temp;
         if(sum<min) min=sum;
         if(sum>max) max=sum;
         if(max-min>=2*M_PI) {
              max=min+2*M_PI;
              break;
         }
     }
     printf("%.2f\n", k*h*(max - min));
     return 0;
}