#include <bits/stdc++.h>
using namespace std;
#pragma warning(disable : 4996)
const int mod = 1e9 + 7;
int epow(int base, int pow) {
  int result = 1;
  while (pow) {
    if (!(pow % 2)) {
      base = (base * base) % mod;
      pow /= 2;
    } else {
      result = (result * base) % mod;
      pow--;
    }
  }
  return result;
}
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  scanf("%d", &n);
  double d = 2 * n * 3.1415926535897932384626;
  cout << d;
}
