#include <bits/stdc++.h>
using namespace std;
void A();
void B();
void C();
void D();
void E();
void F();
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  A();
}
void A() {
  int n;
  cin >> n;
  double cir = n * 2 * 3.14159265358979323846;
  cout << setprecision(8) << cir << endl;
}
void B() {
  int T;
  cin >> T;
  for (int t = 1; t <= T; t++) {
    int N;
    cin >> N;
    int result = 0;
    cout << "Case #" << t << ": ";
    cout << result << endl;
  }
}
void C() {
  int T;
  cin >> T;
  for (int t = 1; t <= T; t++) {
    int N;
    cin >> N;
    int result = 0;
    cout << "Case #" << t << ": ";
    cout << result << endl;
  }
}
void D() {
  int T;
  cin >> T;
  for (int t = 1; t <= T; t++) {
    int N;
    cin >> N;
    int result = 0;
    cout << "Case #" << t << ": ";
    cout << result << endl;
  }
}
void E() {
  int T;
  cin >> T;
  for (int t = 1; t <= T; t++) {
    int N;
    cin >> N;
    int result = 0;
    cout << "Case #" << t << ": ";
    cout << result << endl;
  }
}
void F() {
  int T;
  cin >> T;
  for (int t = 1; t <= T; t++) {
    int N;
    cin >> N;
    int result = 0;
    cout << "Case #" << t << ": ";
    cout << result << endl;
  }
}
