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

void CountCircumReference()
{
	long double circleRadius, circumReference;
	while (1)
	{
		scanf("%Lf", &circleRadius);
		if (circleRadius <= 100 && circleRadius >= 1)
		{
			break;
		}
		printf_s("Wrong input \n");
	}

	circumReference = 2 * M_PI * circleRadius;
	printf("%Lf \n", circumReference);

	return;
}

int main(int argc, const char* argv[])
{
	CountCircumReference();

	return 0;
}