#include <bits/stdc++.h>
const int INF = 1e9;
const int MOD = 1e9 + 7;
using namespace std;
int main(void) {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  cout << 2 * r * 3.14159265 << endl;
  return 0;
}
