#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const int mod2 = 1e9 + 9;
inline int pow_(long long a, int n, int p = mod) {
  int r = 1;
  while (n) {
    if (n & 1) r = r * a % p;
    n >>= 1;
    a = a * a % p;
  }
  return r;
}
int main(void) {
  double R;
  cin >> R;
  double ans = 2 * M_PI * R;
  cout << fixed << setprecision(12) << ans << "\n";
}
