#include <bits/stdc++.h>
using namespace std;
bool aaa[11111111];
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
long long gcd(long long a, long long b) {
  if (a < b) return gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
int main() {
  double xx1, aa;
  aa = 3.1415926535;
  cin >> xx1;
  cout << fixed << xx1 * aa << endl;
}
