#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <map>
#include <algorithm>
#include <iomanip>
#include <queue>
#include <utility>
#include <set>
#include <unordered_set>
#include <stack>
#include <iterator>
#include <random>
#include <fstream>
#include <chrono>
using namespace std;
#define int long long
int m = 1e9 + 7;

vector<int>fact,inv_fact;
void fast(){
    ios_base:: sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
}
int bin_pow(int a,int b){
    if(b==0)
        return 1;
    int res = bin_pow(a,b/2);
    res = (res*res)%m;
    if(b % 2 == 1){
        res= (res*a)%m;
    }
    return res;
}
int inv(int a){
    return bin_pow(a, m-2);
}
int C(int n, int k){
    return (((fact[n]*inv_fact[k])%m)*inv_fact[n-k])%m;
}
int32_t main()
{
    fast();
    double n;
    cin >> n;
    cout << 6.28318530717958623200*n << "\n";
    return 0;
}
