#include <bits/stdc++.h>
#define debug(...) fprintf(stderr, __VA_ARGS__)
using namespace std;
 
void read (int&a) {
	a = 0; char k = getchar(); int f = 1;
	for(;!isdigit(k); k = getchar()) if (k == '-') f = -1;
	for(; isdigit(k); k = getchar()) a = a * 10 + k - '0';
	a *= f;
}

int main ()
{
	double k, pi = acos(-1);
	scanf ("%lf", &k);
	printf ("%.10lf\n", 2 * pi * k);
	return 0;
}