#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t = 1;
  while (t--) {
    long long int r;
    cin >> r;
    cout << 2 * (3.141) * r << endl;
  }
}
