#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vll = vector<long long>;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
constexpr long long mod = 1000000007;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  r = 2 * 3.1415926535 * r;
  printf("%.4lf\n", r);
}
