#include <algorithm>
#include <cmath>
#include <queue>
#include <iomanip>
#include <map>
#include <set>
#include <tuple>
#include <unordered_map>
#include <iterator>
#include <string>
#include <limits>
 
using namespace std;
 
using i64 = int64_t;
using P = pair<i64, i64>;
i64 mod = 1000000007;
int int_max = 2147483647;
int max_n = 1e5;
 
 
int main(int argc, char **argv) {
    double r;
    cin >> r;
 
    cout<< setprecision(10) << r*2.* M_PI <<endl;
 
    return 0;
}