#include <bits/stdc++.h>
using namespace std;
int gcd(int a, int b) {
  if (a % b == 0) {
    return (b);
  } else {
    return (gcd(b, a % b));
  }
}
int main(void) {
  int k;
  double kk;
  double p = 3.141592653589793238462643383279;
  cin >> k;
  kk = (double)k;
  k = 2 * k * p;
  cout << k;
}
