#include <bits/stdc++.h>
using namespace std;
const double PI = 3.141592653589;
const int INF = 1000000007;
const long long LMAX = 1000000000000001;
int gcd(int a, int b) {
  if (a < b) swap(a, b);
  int c = a % b;
  while (c != 0) {
    a = b;
    b = c;
    c = a % b;
  }
  return b;
}
long long lcm(long long a, long long b) { return a * b / gcd(a, b); }
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};
int main() {
  double r;
  cin >> r;
  cout << 2 * r * PI << endl;
  return 0;
}
