第一个：
#include<iostream>
#include<math.h>
#include<stdio.h>
#include<memory>
using namespace std;
const double PI = 3.141592657;
struct Node
{
	double  x,y;
}map[100];
int main()
{
	 
	int testcase,i;
	double  sum = 0.0,rad;
	
	scanf("%d%lf",&testcase,&rad);
	
		 sum = 2*PI*rad;
		 scanf("%lf%lf",&map[0].x,&map[0].y);
	     for(i = 1;i < testcase;i++)
	     {
		      scanf("%lf%lf",&map[i].x,&map[i].y);
			  //cout<<"****";
	      }
	      for( i = 1;i <  testcase;i++)
	      {
		        sum += sqrt((map[i].x - map[i-1].x)*(map[i].x - map[i-1].x)+(map[i].y - map[i-1].y)*(map[i].y - map[i-1].y));
	      }
	      sum += sqrt((map[i-1].x-map[0].x)*(map[i-1].x-map[0].x) + (map[i-1].y - map[0].y)*(map[i-1].y - map[0].y));
	      //cout<<sum<<endl;
	      
	      printf("%.2lf\n",sum);
	
	//system("pause");
	return 0;
	
}
第二个：
#include<iostream>
#include<math.h>
#define PI (3.141592657)
using namespace std;

typedef struct Point{
	double x,y;
}Point;

double LengthOfTwoPoints(Point x,Point y){
	double dx=x.x-y.x,dy=x.y-y.y;
	return sqrt(dx*dx+dy*dy);
}


double CaculateAngle(Point a,Point b,Point c){
	double la = LengthOfTwoPoints(b,c);
	double lb = LengthOfTwoPoints(a,c);
	double lc = LengthOfTwoPoints(a,b);
	return acos((lb*lb+lc*lc-la*la)/(2*lb*lc));
}

double LengthOfArc(double angle,double radius){
	return angle*radius;
}

double LengthOfRope(Point nails[],int N,double radius){
	double totalLength=0;
	int prePos=0,nextPos=0;
	if(N==1) return 2*PI*radius;
	for(int i=0;i<N;i++){
		prePos=i-1,nextPos=i+1;
		if(prePos<0) prePos=N-1;
		if(nextPos>=N) nextPos=0;
		totalLength+=LengthOfTwoPoints(nails[i],nails[nextPos])
			+LengthOfArc(PI-CaculateAngle(nails[i],nails[prePos],nails[nextPos]),radius);
	}
	return totalLength;
}

/*double LengthOfRope(Point nails[],int N,double radius){
	double totalLength=2*PI*radius;
	int nextPos=0;
	if(N==1)
		return totalLength;
	for(int i=0;i<N;i++){
		nextPos=i+1;
		if(nextPos>=N) nextPos=0;
		totalLength+=LengthOfTwoPoints(nails[i],nails[nextPos]);
	}
	return totalLength;
}*/

int main(){
	Point nails[100];
	int N=0;
	double radius=0;
	scanf("%d%lf",&N,&radius);
	for(int i=0;i<N;i++)
		scanf("%lf%lf",&nails[i].x,&nails[i].y);
	printf("%.2lf",LengthOfRope(nails,N,radius));
	return 0;
}
测试数据：
5 1
0 0
1 2
3 2
5 4
3 -1
两个结果不一样啊，都AC了，搞不懂~~~