#include "bits/stdc++.h"
#include <set>
using namespace std;
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define rep(i, n) for (int i = 0; i < (n); i++)
#define rep1(i, n) for (int i = 1; i < (n); i++)
#define rrep(i, n) for (int i = (n)-1; i >= 0; i--)
#define all(i) (i).begin(), (i).end()
#define rall(i) (i).begin(), (i).end(), greater<int>()
#define int long long
#define elif else if
#define eb emplace_back
#define pb push_back
#define mp make_pair
#define fst first
#define sec second
#define mod 1000000007
#define INF 10e10
template <typename T>

void print(T x)
{
	std::cout << x << " ";
}
template <typename T>

void println(T x)
{
	std::cout << x << "\n";
}

int dp[101][101][101];

signed main(void) {
	int R;
	cin >> R;
	std::cout << std::fixed;
	std::cout << std::setprecision(5) << R * 2 * 3.14159265359; 
}
