#include <bits/stdc++.h>
using namespace std;
long long pow3(long long a, long long b, long long m = 1000000007) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = (res * a) % m;
    a = (a * a) % m;
    b >>= 1;
  }
  return res;
}
void solve() {
  long double pi = 3.1415926;
  long long r;
  cin >> r;
  cout << setprecision(10) << 2.0 * pi * r;
}
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long q = 1;
  for (long long i = 0; i < q; i++) {
    solve();
  }
  return 0;
}
