#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<vector>
#include<functional>
#include<algorithm>
#include<stdlib.h>
#include<string.h>
#include<cmath>
#include<deque>
#include<set>
using namespace std;
typedef long long ll;
#define rep(i,a,b) for(int i=a;i<b;i++)
#define all(_x) _x.begin(), _x.end()
#define r_sort(_x) sort(_x.begin(), _x.end(), std::greater<int>())
#define INF 1LL << 60
#define MAX 1000000000
const double PI = 3.14159265358979323846;
int gcd(int a, int b) { return a % b == 0 ? b : gcd(b, a % b); }
int main() {
	int R;
	scanf("%d", &R);
	printf("%f", 2 * R * PI);
	return 0;
}