#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

int main(int argc, char *argv[]){
        int r;
        double pi = 3.1415926535897932;
        double ans;
        scanf("%d",&r);
        ans = 2 * r * pi;
        printf("%lf",ans);
        return 0;
}