#include <bits/stdc++.h>
int main(void) {
  int rect = 0;
  float ans = 0.0f;
  char buf[101];
  memset(buf, 0x00, sizeof(buf));
  fgets(buf, sizeof(buf), stdin);
  sscanf(buf, "%d", &rect);
  printf("rect:%d\n", rect);
  ans = 2 * (float)rect * 3.1415;
  printf("%lf", ans);
  return 0;
}
