#include <bits/stdc++.h>
#define ALL(a)  (a).begin(),(a).end()
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
typedef pair<long long, long long> Pll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<long long> vll;
typedef vector<vector<long long>> vvll;
template <typename T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template <typename T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const long long INF = 1LL << 60;
const int INT_INF = 1 << 30;
#define MOD 1000000007LL

#define PI 3.14159265358979

int main(){
	double R;
	cin >> R;
	cout << 2 * R * PI << endl;
}