#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
const long long INF = 1e16;
const long long MAX_N = 100010;
double pi = acos(-1);
int main() {
  double r;
  cin >> r;
  double ans = 2.0 * pi * r;
  printf("%.10f\n", ans);
  return 0;
}
