#include <cstdlib>
#include <iterator>
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cctype>
#include <functional>
#include <cmath>
#include <stack>
#include <bitset>

using namespace std;

#define int long long
#define MAX 1e18

signed main() {
    int r; cin >> r;
    cout << 2 * M_PI * r << endl;

}