#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <cmath>
#include <stack>
#include <queue>
#include <set>

#define mod 1000000007
#define mp make_pair
#define rep(i,n) for(int i=0;i<n;i++)
#define _GLIBCXX_DEBUG
#define _LIBCPP_DEBUG 0

using namespace std;
using ll = long long;
using pii = pair<int, int>;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n;
    cin >> n;
    cout << 2 * n * M_PI;
    return 0;
}