#include <bits/stdc++.h>
inline int read() {
  register int x = 0;
  register int y = 1;
  register char c = std::getchar();
  while (c < '0' || c > '9') {
    if (c == '-') y = 0;
    c = std::getchar();
  }
  while (c >= '0' && c <= '9') {
    x = x * 10 + (c ^ 48);
    c = std::getchar();
  }
  return y ? x : -x;
}
int main() {
  int r = read();
  std::printf("%lf", 2 * 3.1415926 * r);
  return 0;
}
