#include <iostream.h>
#include <math.h>

struct tPt{double x,y;}p1,p2,fs;

int n;
double dx,dy,r,l;
const double PI=3.141592;

int main()
{
  cin>>n>>r>>p1.x>>p1.y;
  fs=p1;
  l=2*PI*r;
  for (int i=1;i<n;i++)
  {
    cin>>p2.x>>p2.y;
    dx=p2.x-p1.x;dy=p2.y-p1.y;
    l+=sqrt(dx*dx+dy*dy);
    p1=p2;
  }
  if (n>1)
  {
    dx=p1.x-fs.x;dy=p1.y-fs.y;
    l+=sqrt(dx*dx+dy*dy);
  }
  cout.precision(2);
  cout<<l;
  return 0;
}

try this:

l=3.00000
cout.precision(2);
cout<<l;

the result is 3 (not 3.00)

that after using cout, use printf for output float and double with
precision.