#include <bits/stdc++.h>
using namespace std;
vector<int> vec;
int gcd(int x, int y) { return (x % y) ? gcd(y, x % y) : y; }
int main(void) {
  double a;
  long long ans;
  ans = 0;
  cin >> a;
  cout << a * 3.1415;
}
