#include <bits/stdc++.h>
void CountCircumReference() {
  long double circleRadius, circumReference;
  while (1) {
    printf("Enter the radius of the cirlce \n");
    scanf("%Lf", &circleRadius);
    if (circleRadius <= 100 && circleRadius >= 1) {
      break;
    }
    printf("Wrong input \n");
  }
  circumReference = 2 * M_PI * circleRadius;
  printf("%Lf \n", circumReference);
  return;
}
int main(int argc, const char* argv[]) {
  CountCircumReference();
  return 0;
}
