#include <bits/stdc++.h>
using namespace std;
bool compPair(const pair<int, int>& arg1, const pair<int, int>& arg2) {
  return arg1.first > arg2.first;
}
template <class T>
void chmax(T& a, T b) {
  if (a < b) {
    a = b;
  }
}
template <class T>
void chmin(T& a, T b) {
  if (a > b) {
    a = b;
  }
}
int main(void) {
  double r;
  cin >> r;
  cout << 2 * 3.1415926535 * r << endl;
  return 0;
}
