#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7LL;
const int INF = 2e9;
double N;
int main() {
  cin.tie(0), cout.tie(0), ios::sync_with_stdio(0),
      cout << fixed << setprecision(20);
  ;
  cin >> N;
  cout << 2 * N * 3.14159265358979 << "\n";
  return 0;
}
