#include <stdio.h>
#include <math.h>
#include <string.h>

#define PI  3.1415
#define BUF_MAX 101    

int main(void)
{
    int     rect = 0;
    float   ans = 0.0f;
    char    buf[BUF_MAX];
    
    memset(buf, 0x00, sizeof(buf));
    
    fgets(buf, sizeof(buf), stdin);
    sscanf(buf, "%d", &rect);
    
    ans = 2 * (float)rect * PI;
    
    printf("%lf", ans);
    
    return 0;
}
