#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for( ll i = 0;i < (ll) n;++i)

int main(){
  double r;
  cin >> r;
  printf("%.9lf\n",r*2.0*3.141592653589793238);
  return 0;
}