#include<stdio.h>
#define
float getCircumference(float radius){
    return 2*PI*radius;
}
int main(){
    float radius;
    scanf("%f",&radius);
    if(radius<0){
        printf("Invalid input");
        return 0;
    }
    float circumference=getCircumference(radius);
    printf("%.2f",circumference);
    return 0;
}