#include <bits/stdc++.h>
using namespace std;
const int inf = 1e+9;
const long long mod = 1000000007;
const double PI = 3.14159265358979323846;
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
int cans(bool f) {
  if (f)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
  return 0;
}
int main() {
  float l;
  cin >> l;
  cout << 2 * PI * l << endl;
  return 0;
}
