#include <bits/stdc++.h>
using namespace std;
void read(int& a) {
  a = 0;
  char k = getchar();
  int f = 1;
  for (; !isdigit(k); k = getchar())
    if (k == '-') f = -1;
  for (; isdigit(k); k = getchar()) a = a * 10 + k - '0';
  a *= f;
}
int main() {
  double k, pi = acos(-1);
  scanf("%lf", &k);
  printf("%.10lf\n", 2 * pi * k);
  return 0;
}
