#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector> 
#include <cstring>
#include <queue>
#include <ctime>
#include <set>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,int> pli;
const ll INF = 10e16;
const int maxN = 200010;
const int maxD = 15;
const int maxH = 100; 
const int MOD = 1000000007;


vector<string> ans;
vector<int> p;


int main()
{
	double p = 3.1415926;
	int R;
	cin>>R;
	double ans = p * R * 2;
	printf("%f\n",ans); 
	return 0;
}