#define __STDC_WANT_LIB_EXT1__
#include <stdio.h>
#include <math.h>
 
void StartProgram()
{
    long double circleRadius, circumReference;
    scanf("%Lf", &circleRadius);
    
    circumReference = 2 * M_PI * circleRadius;
    printf_s("%Lf", circumReference);
    
    return ;
}
 
int main(int argc, const char * argv[])
{
    StartProgram();
    
    return 0;
}