#include <bits/stdc++.h>
int main() {
  int R;
  double c;
  printf("Enter radius \n");
  scanf("%d", &R);
  if (R > 100 || R < 1) {
    printf("Wrong Input");
    return 0;
  }
  c = 2 * 3.141592653589793238 * (double)(R);
  printf("Radius=%.15lf", c);
}
