/*
 * FileName:     A
 * CreatedDate:  2020-04-19 20:57:47 +0900
 * LastModified: 2020-04-19 21:05:11 +0900
 */

#include <stdio.h>

int main(void){
    double R;
    scanf("%lf",&R);
    printf("%lf\n",2*3.1415926*R);
    return 0;
}
