#include <bits/stdc++.h>
using namespace ::std;
int ri() {
  int x;
  scanf("%d", &x);
  return x;
}
int64_t ri64() {
  int64_t x;
  scanf("%lld", &x);
  return x;
}
void wi(int x) { printf("%d\n", x); }
void wvi(vector<int> &v) {
  for (int i = 0; i < v.size(); i++) printf("%d ", v[i]);
  printf("\n");
}
void dbg(string &str, int x) {
  cout << str << ": ";
  wi(x);
}
void dbg(string &str, vector<int> &x) {
  cout << str << ": ";
  wvi(x);
}
int main() {
  long double pi = 4.0 * atanl(1.0);
  long double R;
  cin >> R;
  long double ans = 2.0 * pi * R;
  printf("%.2f\n", (double)ans);
  return 0;
}
