#include <iostream>
#include <stdio.h>
#include <stdint.h>
#include <vector>
#include <string>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <string>
#include <math.h>
#include <time.h>
#include <algorithm>
#include <deque>

using namespace::std;

int ri()
{
	int x;
	scanf("%d", &x);

	return x;
}

int64_t ri64()
{
	int64_t x;
	scanf("%lld", &x);

	return x;
}

void wi(int x)
{
	printf("%d\n", x);
}

void wvi(vector<int> &v)
{
	for (int i = 0; i < v.size(); i++)
		printf("%d ", v[i]);
	printf("\n");
}

void dbg(string &str, int x)
{
	cout << str << ": ";
	wi(x);
}

void dbg(string &str, vector<int> &x)
{
	cout << str << ": ";
	wvi(x);
}

int main()
{
        long double pi = 4.0 * atanl(1.0);
        
        long double R;
        cin >> R;

        long double ans = 2.0 * pi * R;
        printf("%.2f\n", (double) ans);

	return 0;
}
