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

void StartProgram()
{
    long double circleRadius, circumReference;
    scanf("%Lf", &circleRadius);
    
    circumReference = 2 * M_PI * circleRadius;
    printf("%Lf", circumReference);
    
    return ;
}

int main(int argc, const char * argv[])
{
    StartProgram();
    
    return 0;
}
