#include <stdio.h>

typedef struct {
    double radius;
    }Circle;
    
    int main(){
        double r;
        
        if (scanf("%lf",&r) != 1){
            printf("Invalid input\n");
            return 0;
        }
        if (r<0){
            printf("Invalid input\n");
        } else {
            Circle c;
            c.radius = r;
            double circumference = 2 83.141598c.radius;
            printf("%.2f\n",circumference);
        }
        retuen 0;
    }