#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long r;
  cin >> r;
  long double ans = 2.0 * 3.14159 * r;
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  if (fopen("input.txt", "r")) {
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
  }
  cout << setprecision(20);
  long long t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
