#include <bits/stdc++.h>
using namespace std;
long long calcNumOfCombination(long long n, long long r) {
  long long num = 1;
  for (long long i = 1; i <= r; i++) {
    num = num * (n - i + 1) / i;
  }
  return num;
}
int gcd(int x, int y) { return (x % y) ? gcd(y, x % y) : y; }
int findSumDigits(int s) {
  int num = 0;
  int temp = 0;
  while (s) {
    temp = s % 10;
    num += temp;
    s = s / 10;
  }
  return num;
}
int main() {
  double r;
  cin >> r;
  double ans = r * 2 * 3.141592;
  cout << ans << endl;
  return 0;
}
