感谢感谢

观察一下，发现，最终绳子的长度=一个圆周长度+圆心逆时针首尾相连得到的多边形周长。

/*

*/
#define method_1
#ifdef method_1
/*

*/
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<vector>
#include<cstring>
#include<cstdlib>
#include<iomanip>
#include<ctime>
#include<string>
#include<bitset>
#define D(x) cout<<#x<<" = "<<x<<"  "
#define E cout<<endl
using namespace std;
typedef long long ll;
typedef pair<int,int>pii;
const int maxn=100+5;
const int INF=0x3f3f3f3f;
const double pi=acos(-1.0);
int n;
double r;
double ans=0.0;
double x[maxn],y[maxn];
double dis(int i,int j){
    return sqrt((x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j]));
}
void solve(){
    ans+=2*pi*r;
    for(int i=1;i<=n;i++){
        scanf("%lf%lf",&x[i],&y[i]);
        if(i==1) continue;
        ans+=dis(i,i-1);
    }
    ans+=dis(n,1);
}
int main() {
    ios::sync_with_stdio(false);
//  freopen("绕钉子的长绳子.in","r",stdin);
    scanf("%d%lf",&n,&r);
    solve();
    printf("%.2lf",ans);
    return 0;
}
#endif
#ifdef method_2
/*

*/

#endif
#ifdef method_3
/*

*/

#endif