#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline T read() {
  T x = 0, f = 1;
  char c = getchar();
  while (!isdigit(c)) {
    if (c == '-') f = -f;
    c = getchar();
  }
  while (isdigit(c)) {
    x = (x << 3) + (x << 1) + (c - 48);
    c = getchar();
  }
  return x * f;
}
const int inf = 1e9 + 1e7;
const double PI = 3.1415926;
int main(void) {
  double n;
  cin >> n;
  cout << 2 * n * PI << endl;
  return 0;
}
