#include<bits/stdc++.h> 
using namespace std; 
int n,a[2000000],b[2000000]; 
double s,t;
double far(int a,int b,int c,int d){
	return sqrt((a-c)*(a-c)*1.0+(b-d)*1.0*(b-d));
}
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
		t=far(a[i],b[i],a[i-1],b[i-1]);
		s+=t; 
	}
	s+=far(a[n+1],b[n+1],a[n],b[n]);
	printf("%.12lf",s);
	return 0;
}

题目说误差不超过$10^{-6}$就行了

验证码：wwww