#include<iostream>
#include<string>
#include<algorithm>    
#include<cmath>
#include<map>
#include<vector>
#include<math.h>
#include<stdio.h>
#include<stack>
#include<queue>
#include<tuple>
#include<cassert>
#include<set>
//#include<bits/stdc++.h>
#define int long long
using namespace std;
const int INF = 1000000000000000000;
const int mod = 1000000007;
int test[345678], aa[345678][2];
signed main() {
	int n, m, cnt = 0, ans = 0; string s, t; 
	cin >> n;
	printf("%f\n", n * 2 * 3.141592);
	return 0;
}