#include <string>
#include <map>
#include <vector>
#include <algorithm>
#include <iostream>
#include <string>
#include <map>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;
#define ll long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define Graph vector<vector<int>>;
#define iterG(next_v, G, v) for(auto next_v : G[v]
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int r;
    cin >> r;
    cout << 2 * r * M_PI << endl;
    return 0;
}

