#include <bits/stdc++.h>
using namespace std;
void inversemod(long long int n, long long int mod, long long int invmod[],
                long long int MOD) {
  for (int i = 2; i <= n; ++i)
    invmod[i] = MOD - MOD / i * invmod[MOD % i] % MOD;
}
int main() {
  double n, m, h;
  string s;
  cin >> n;
  cout << setprecision(15) << fixed << 2 * acos(-1.0) * n << endl;
}
