#include <bits/stdc++.h>
using namespace std;
const long long int MOD = 1000000007;
const long long int MAX_N = 500010;
const long long int INF = 9999999999;
int main() {
  long long int r;
  long double pi = 3.1415926535;
  cin >> r;
  cout << setprecision(10) << 2 * pi * r;
  return 0;
}
