#include <bits/stdc++.h>
using namespace std;
inline int ctoi(char c) {
  if (c < '0' || '9' < c) throw invalid_argument("ctoi error");
  return c - '0';
}
template <typename T>
inline T gcd(T x, T y) {
  if (x <= 0 || y <= 0) throw invalid_argument("gcd error: x <= 0 or y <= 0");
  if (x < y) swap(x, y);
  T r = x % y;
  while (r != 0) {
    x = y;
    y = r;
    r = x % y;
  }
  return y;
}
template <typename T>
inline T lcm(T x, T y) {
  if (x <= 0 || y <= 0) throw invalid_argument("lcm error: x <= 0 or y <= 0");
  return x * y / gcd(x, y);
}
int main() {
  int r;
  cin >> r;
  cout << 2 * 3.14159265359 * r;
  return 0;
}
