#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<long>;
using vvi = vector<vi>;
using vs = vector<string>;
template <typename T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  char r;
  cin >> r;
  cout << r * 3.141592 << endl;
  return 0;
}
