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

int cal(double x1, double y1, double x2, double y2){
    int d;
    d=sqrt(pow((x2-x1),2)+pow((y2-y1),2));
    return d;
}

int main(){
  int no_of_nails,j;
  double radius;
  double total_length=0;
  double x1,y1,x2,y2;
  double nextx,nexty,prevx,prevy;

  fscanf(stdin, "%d %lf",&no_of_nails,&radius);
  getchar();

  total_length += no_of_nails * ((M_PI*radius)/2);

  if(no_of_nails==1)  {
    total_length += M_PI*2*radius;
    printf("%0.2lf",total_length);
    return 0;
  }


  fscanf(stdin,"%lf %lf",&x1,&y1);
  fscanf(stdin,"%lf %lf",&x2,&y2);

  total_length+=cal(x1,y1,x2,y2);

  prevx=x2;
  prevy=y2;
  if(no_of_nails>2){
      for(j=no_of_nails-2;j>0;j--){
          fscanf(stdin,"%lf %lf",&nextx,&nexty);
          total_length += cal(prevx,prevy,nextx,nexty);
          prevx=nextx;
          prevy=nexty;
      }
      total_length += cal(prevx,prevy,x1,y1);
  }

  printf("%0.2lf",total_length);
  return 0;


}

understood my mistake, 2pir :) got AC.