#include<iostream>
#include<string>
#include<stdio.h>
#include<cctype>
#include<algorithm>
#include<climits>
#include<cmath>
#include<map>
#include<vector>
#define INF INT_MAX
using namespace std;

int r;
string s;
double ans;


int main() {
	cin >> r;
	
	ans = (r + r) * 3.141592;

	cout << ans << endl;

	return 0;
}