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

//A	Circle Pond	2 sec	1024 MB	提出
int Question163A(void) {
	float input;
	float output;
	scanf("%f", &input);
	output = 2 * input*M_PI;
	printf("%.2f\n", output);
}
