#include <bits/stdc++.h>
using namespace std;
long long int power(long long int x, long long int y) {
  long long int res = 1;
  x = x;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
long long int logtwo(long long int n) {
  if (n == 1) return 0;
  return logtwo(n / 2) + 1;
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    long long int r;
    cin >> r;
    cout << fixed << setprecision(5) << 2 * 3.1415 * r << "\n";
  }
}
