In Visual C++ there is now constant pi, so I use constant from Builder5 pi=3.14159265358979323846.
I also tryed 2*acos(0) but still WA.
Someone please help me if you can.

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

#define pi 3.14159265358979323846

void main(){
    int N, R;
    cin>>N>>R;
    int i;
    double t1_x, t2_x, tf_x, t1_y, t2_y, tf_y;
    cin>>tf_x>>tf_y;
    t1_x=tf_x;
    t1_y=tf_y;
    double sum_l=0;
    for(i=1; i<N; i++){
        cin>>t2_x>>t2_y;
        sum_l+=sqrt((t2_x-t1_x)*(t2_x-t1_x)+(t2_y-t1_y)*(t2_y-t1_y));
        t1_x=t2_x;
        t1_y=t2_y;
    }
    if (N>1)
        sum_l+=sqrt((t2_x-tf_x)*(t2_x-tf_x)+(t2_y-tf_y)*(t2_y-tf_y));

    int sum=int((sum_l+2*R*pi)*100);
    cout<<double(sum)/100;
}

You should also remember about additional length of rope:
rope doesn't have only one common point with nail (in general case)!