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

typedef pair<int,int> P;
#define ll long long
#define ld long double
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define PI 3.14159265358979323846
#define sz(x) ((int)(x).size())
#define chmin(x,y) x = min(x,y)
#define chmax(x,y) x = max(x,y)

const int INF = 1e9;
const int MOD = 1e9 + 7;
const ll LINF = 1e18;
int main()
{
    double r;
    cin >> r;
    cout << setprecision(11) << (double)2*r*PI << endl;
}