#include<stdio.h>
int main(void) {
     float radius;
     if(scanf("%f",&radius)!=1){
         printf("Invalid input");
         return 0;
         
     }
if (radius <=0){
    printf("Invalid input");
    return 0;
}
float circumference = 2.0f * 3.14159f * radius:
printf("%2f", circumference);
return 0;
}