#include<stdio.h>
#define
struct circle{
    float radius;
    };
    float calculatecircumference(struct circle c){
        return 2 * PI * c.radius;
    }
    int main(){
        struct circle c;
        scanf("%f",&c.radius);
        if(c.radius<=0 || c.radius>1000){
            printf("Invalid input");
        }else{
            printf("%.2f",calculatecircumference(c));
        }
     return 0;
    }