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

   int ix=1,iy=1;

void main(){

   int i,n;
   double area=0.0;
   double w,f,g,x,rl,ru;
   double pi=3.141592653589793238462643383279502884197;
   double urnd(void);

   for(i=1;i<=10000;i++) urnd();

   scanf("%lf%lf%d",&rl,&ru,&n);

   for(i=1;i<=n;++i){
     x=(ru-rl)*urnd()+rl;
     f=exp(-0.5*x*x)/sqrt(2.0*pi);
     g=1.0/(ru-rl);
     w=f/g;
     area+=w/((double)n);
   }
   printf("%10.8lf\n",area);
}
/* ================================================ */
double urnd(void)
{
      int    kx,ky;
      double rn;
/*
   Input:
     ix, iy:  Seeds
   Output:
     rn: Uniform Random Draw U(0,1)
*/
      kx=ix/53668;
      ix=40014*(ix-kx*53668)-kx*12211;

      ky=iy/52774;
      iy=40692*(iy-ky*52774)-ky*3791;

      rn=(double)(ix-iy)/2147483563.;
      rn-=(int)rn;
      if( rn<0.) rn++;

      return rn;
}