#include <bits/stdc++.h>
using namespace std;
int vx[] = {0, 0, -1, 1};
int vy[] = {1, -1, 0, 0};
int dx[] = {1, 0, -1, 1, -1, 1, 0, -1};
int dy[] = {1, 1, 1, 0, 0, -1, -1, -1};
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a * b / gcd(a, b); }
int main() {
  double r;
  cin >> r;
  double pi = 3.14159265;
  cout << r * pi << endl;
  return 0;
}
