#include <bits/stdc++.h>
using namespace std;
int max(int a, int b) {
  int max;
  if (a > b)
    max = a;
  else
    max = b;
  return max;
}
int min(int a, int b) {
  int min;
  if (a < b)
    min = a;
  else
    min = b;
  return min;
}
int main() {
  double p = acos(-1);
  int r;
  cin >> r;
  cout << p * r << endl;
}
