#include <bits/stdc++.h>
using namespace std;
const long long logN = 20;
const long long M = 1000000007;
const long long INF = 1e12;
const long long N = 100005;
void solve() {
  long double r;
  cin >> r;
  cout << fixed << setprecision(6) << 2 * 3.14159265 * r << "\n";
  return;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long no;
  no = 1;
  while (no--) {
    solve();
  }
  return 0;
}
