#include <stdio.h>

#define _USE_MATH_DEFINES
#include <math.h>

int main(void) {

	int R;
	double result;

	scanf("%d", &R);

	if (R >= 1, R <= 100) {

		result = R * 2 * M_PI;

		printf("%lf", result);
	}

	return 0;

}