#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
const long long INF = (1LL << 62);
int main() {
  std::cout << std::fixed << std::setprecision(10);
  long long a;
  scanf("%lld", &a);
  cout << (a * 2 * 3.1415926535) << endl;
}
