#include <bits/stdc++.h>
using namespace std;
long long N, A, B;
pair<long long, long long> max_element(long long n) {
  long long ele = 1;
  long long lim = (long long)sqrt(n);
  for (long long i = 1; i <= lim; i++) {
    if (!(n % i)) {
      ele = i;
    }
  }
  pair<long long, long long> res = {ele, n / ele};
  return res;
}
int digitnum(long long n) {
  int res = 1;
  while (n / 10) {
    res++;
    n /= 10;
  }
  return res;
}
int main() {
  cin >> N;
  printf("%lf\n", N * M_PI);
  return 0;
}
