#include <iostream>
#include <string>
#include <vector>

#define rep(i,n) for(int i=0; i <(n); i++)

using namespace std;

int main(void){

    //input
    int R;
    cin >> R;

    //output
    int arc = 0;

    cout<< 2*R*3.14;
    
    return 0;
}
