#include <algorithm>
#include<iostream>
#include<stdio.h>
#include<iostream>
#include<stdio.h>
#include<vector>
#include<set>
#include<string>
#include<iomanip>
#include<queue>
#include<functional>
#include<cmath>
#include<map>
#define rep(i,n) for(int i=0; i<(int)n; i++)//n回繰り返す
using namespace std;
using ll = long long;
using P = pair<int, int>;
using graph = vector<vector<int>>;


int main() {
    int N; cin >> N;
    double ans = 2 * (double)N * 3.14;
        cout<< ans << endl;
}

    


    






































































































































