#include <bits/stdc++.h>
int main() {
  int R;
  long double c;
  printf("enter the radius of the circle--");
  scanf("%d", &R);
  printf("\n");
  if (R >= 1 || R <= 100) {
    c = 2 * 3.14159 * R;
    printf("%Lf", c);
  } else {
    printf("enter the valid radius");
  }
  return 0;
}
