#include <bits/stdc++.h>
using namespace std;
template <class _T>
inline void read(_T &_a) {
  bool f = 0;
  char _c = getchar();
  _a = 0;
  while (_c < '0' || _c > '9') {
    if (_c == '-') f = 1;
    _c = getchar();
  }
  while (_c >= '0' && _c <= '9') {
    _a = (_a << 3) + (_a << 1) - '0' + _c;
    _c = getchar();
  }
  if (f) _a = -_a;
}
const double pi = 3.1415926;
double r;
int main() {
  scanf("%lf", &r);
  printf("%.2lf", 2 * pi * r);
  return 0;
}
