#include <bits/stdc++.h>
using namespace std;
struct edge {
  long long to, cost;
};
long long ceil2(long long a, long long b) {
  if (a % b) {
    return a / b + 1;
  } else {
    return a / b;
  }
}
using namespace std;
long long n, k, ans;
long long x, y;
signed main() {
  double n;
  cin >> n;
  cout << n * 3.14 * 2.0 << endl;
}
