#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
const int MAX = 500000;
const int inf = 1000000007;
const long long mod = 1000000007;
const long long longinf = 1LL << 60;
const long double PI = (acos(-1));
const long double EPS = 0.0000000001;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
double a;
int main() {
  cin >> a;
  cout << a * PI * 2 << endl;
  return 0;
}
