#include <bits/stdc++.h>
using namespace std;
void fopen() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
void solve() {
  long double r;
  cin >> r;
  cout << 2 * 3.14159265358979323846 * r;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  int t;
  for (int i = 0; i < 1; i++) {
    solve();
    cout << endl;
  }
}
