#include <bits/stdc++.h>
using namespace std;
 
int ctoi(const char c){
  switch(c){
    case '0': return 0;
    case '1': return 1;
    case '2': return 2;
    case '3': return 3;
    case '4': return 4;
    case '5': return 5;
    case '6': return 6;
    case '7': return 7;
    case '8': return 8;
    case '9': return 9;
    default : return -1;
  }
}

#define INF 1e+9




int main(){

ios::sync_with_stdio(false);
cin.tie(0);

double n;
cin>>n;
cout << fixed << setprecision(15) <<(n*2)*M_PI << endl;

}
