#include <bits/stdc++.h>
using namespace std;

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

int main(){
    double R;
    cin>>R;
    double ans=2.0*R*3.141592;
    cout<< std::setprecision(3) <<ans<<endl;
    return 0;
}