#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const long long infl = 0x3f3f3f3f3f3f3f3fLL;
const int infi = 0x3f3f3f3f;
void solve() {
  long double r;
  cin >> r;
  cout << r * M_PIl * 2;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
}
