#include<bits/stdc++.h>
using namespace std;

#define ALL(V) (V).begine(),(V).end()
#define SORT(V) sort(ALL(V))
#define REV(V) reverse(ALL(V))
#define RSORT(V) SORT(V);REV(V)

#define rep(i,n) for(int i = 0;i < (int)(n);++i)
#define Rep(i,n) for(int i = 1;i <= (int)(n);++i)

template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b;return true;}return false;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b;return true;}return false;}

#define SZ(x) ((int)(x).size())

typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;

double PI = 3.14;

/* 入力 */
int N,M;

void calc()
{
    cin >> N;
    cout << (double)N * 2 * PI << endl;
    return;
}

int main()
{
    calc();
}