#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using uint = unsigned int;
using ldouble = long double;
int ctoi(char c) {
  switch (c) {
    case '0':
      return 0;
    case '1':
      return 1;
    case '2':
      return 2;
    case '3':
      return 3;
    case '4':
      return 4;
    case '5':
      return 5;
    case '6':
      return 6;
    case '7':
      return 7;
    case '8':
      return 8;
    case '9':
      return 9;
    default:
      cout << "ERR_ctoi" << endl;
      return -1;
  }
}
int main() {
  int x;
  cin >> x;
  cout << (double)x * 3.1415926535 << endl;
  return 0;
}
