#include <bits/stdc++.h>
int main() {
  int R;
  float circumference;
  printf("enter radius: ");
  scanf("%d", &R);
  circumference = 2 * 3.1416 * R;
  if (1 <= R && R <= 100)
    printf("circumference of a cycle=%f", circumference);
  else
    return 0;
}
