#include <bits/stdc++.h>
using namespace std;
long long modpow(long long base, long long exp, long long modulus) {
  base %= modulus;
  long long result = 1;
  while (exp > 0) {
    if (exp & 1) result = (result * base) % modulus;
    base = (base * base) % modulus;
    exp >>= 1;
  }
  return result;
}
const long double pi = acos(-1);
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long double r;
  cin >> r;
  long double ans = 2 * pi * r;
  cout << ans;
  return 0;
}
