#include <bits/stdc++.h>
using namespace std;
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
template <typename T>
T POW(T B, T P) {
  if (P == 0) return 1;
  if (P & 1)
    return B * POW(B, P - 1);
  else
    return (POW(B, P / 2)) * (POW(B, P / 2));
}
long long powmod(long long a, long long b) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % 100000007;
    a = a * a % 100000007;
    b >>= 1;
  }
  return ans;
}
#pragma GCC target("avx2")
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
int main() {
  double r;
  cin >> r;
  double res = 2 * acos(-1) * r;
  cout << fixed << setprecision(20) << res << '\n';
}
