#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i < (n + 1); ++i)
using namespace std;
using ll = long long;
const int INF = +100100100;
// typedef pair<int,int> P;
const int MAX = 1000000;

int main(){
    int r;
    cin >> r;
    cout << r*2* M_PI << endl;
    return 0;
}