#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using plong = pair<ll, ll>;
ll MOD = 998244353;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
int main() {
  double A;
  cin >> A;
  double res = A * 2 * 3.1415926535;
  printf("%.10f\n", res);
}
