#include <bits/stdc++.h>
using namespace std;
int rd() {
  int ans = 0, sign = 1;
  char c = getchar();
  while (!(('0' <= (c) && (c) <= '9') || (c) == '-')) c = getchar();
  if (c == '-') sign = -1, c = getchar();
  while ((('0' <= (c) && (c) <= '9') || (c) == '-'))
    ans = (ans << 3) + (ans << 1) + c - '0', c = getchar();
  return sign == 1 ? ans : -ans;
}
int r;
double ans = acos(-1);
int main() {
  r = rd();
  ans *= r;
  printf("%.20lf\n", ans);
  return 0;
}
