#include <bits/stdc++.h>
using namespace std;
long long mod = 1e9 + 7;
int gcd(int x, int y) {
  if (x < y) swap(x, y);
  int r = x % y;
  if (r == 0) {
    return y;
  }
  return gcd(y, r);
}
long long mypow(long long x, long long n) {
  if (n == 0) return 1;
  if (n % 2 == 0)
    return mypow(((x % mod) * (x % mod)) % mod, n / 2);
  else
    return ((x % mod) * mypow(((x % mod) * (x % mod)) % mod, (n - 1) / 2)) %
           mod;
}
int main() {
  double R;
  cin >> R;
  cout << fixed << setprecision(16);
  cout << 2.0 * R * M_PI << endl;
}
