#include <algorithm>
#include <cmath>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

int main(void){
    double r;
    std::cin >> r;
    std::cout >> r * 3.141592653589793238462 >> std::endl;

    return 0;
}